Control apparatus and method for optical amplifier, optical amplifier, optical transmission apparatus, individual band gain equalizer, wavelength multiplexing transmission apparatus, optical amplifier and wavelength multiplexing transmission system using the same equalizer

ABSTRACT

A control apparatus comprises a light monitoring unit for dividing a signal wavelength band into at least a band in which output light power of an optical amplifier tends to decrease at an decrease in the number of signal wavelengths and a band including a gain deviation band, and for monitoring inputted light power for the individual divided bands, a calculation unit for obtaining the number of signal wavelengths in the individual divided bands based on a monitor result, and a target gain correction unit for correcting a target gain based on a result of the calculation. This suppresses a transient variation of signal light level due to SHB or SRS at a high speed with a simple configuration without deteriorating noise characteristic, thus enabling optical amplifiers to be further disposed in a multi-stage fashion, which can lengthen the transmission distance of a transmission system including an optical add/drop unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to JapaneseApplications No. 2005-286608 filed on Sep. 30, 2005 and No. 2005-71044filed on Mar. 14, 2005 in Japan, each of the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a control apparatus and method for anoptical amplifier, an optical amplifier, an optical transmissionapparatus, an individual band gain equalizer, a wavelength multiplexingtransmission apparatus, an optical amplifier and a wavelengthmultiplexing transmission system using the same equalizer, and moreparticularly to a technique suitable for use in a WDM (WavelengthDivision Multiplex) optical transmission system which lengthens thetransmission distance by disposing optical fiber amplifiers, representedby an Erbium-doped fiber amplifier (EDFA), in multi-stage system.

(2) Description of the Related Art

In recent years, as one example of a WDM optical transmission system,there has been noted a metro core system capable of adding/dropping anoptical signal with an arbitrary wavelength at arbitrary nodes making aconnection between local base cities. FIG. 16 is a block diagram showingan example of a configuration of a metro core system. In a system shownin FIG. 16, a plurality of OADM (Optical Add-Drop Multiplexer) nodes 100are connected through transmission lines (optical fibers) 400 into aring configuration so that, at each of the OADM nodes 100, a signal withan arbitrary wavelength (channel) is added to the transmission line 400and, of WDM signals transmitted through the transmission line 400, lightwith an arbitrary wavelength is dropped therefrom. Moreover, opticalamplifiers (preamplifier 200 and post amplifier 300) are properly placedat the former and latter stages of each of the OADM nodes 100, therebycompensating for the loss in light level between the OADM nodes (each ofwhich will hereinafter be equally referred to simply as a “node”) 100for lengthening the transmission distance.

In addition, in the system which adds/drops a signal at an arbitrarynode 100 as mentioned above, since the number of signal wavelengths(hereinafter referred to equally as the “number of transmissionwavelengths”) to be transmitted in the system (transmission lines 400)varies dynamically, for maintaining the output signal power for eachwavelength (channel) to a constant value with respect to this variationof number of wavelengths (keeping the flatness of gain with respect towavelength), an AGC amplifier having an automatic gain control (AGC)function is commonly used for the above-mentioned amplifiers 200 and300.

In this case, for example, as shown in FIG. 17, the AGC amplifier isarranged such that portions of input/output signals of an opticalamplifier (EDFA) 200 (300) are branched through the use of opticalbranch means 501 and 502 such as optical couplers so as to monitor thepowers thereof (i.e., input/output signal powers of the opticalamplifier 200 (300)) by PDs 601 and 602, and an automatic gain controlunit 700 controls the pump power of the EDFA 200 (300) so that the powerratio therebetween becomes constant.

Meanwhile, in such a system, it is considered that, for example, asshown in FIG. 18(A), many (for example, 39 wavelengths) optical signalsare added from one node 100 (100A) and a different one-wavelengthoptical signal is added from the next node 100 (100B) . In thissituation, for example, as shown in FIG. 18(B), in a case in which atrouble such as dynamic reconstruction of an optical transmission path,man-made mistake, fiber disconnection and connector removal occursbetween the nodes 100A and 100B, only the signal added at the node 100Bremains (that is, the number of transmission wavelengths variessuddenly. At this time, for example, as shown in FIG. 19(A), therearises a phenomenon that the power level of the residual light at thelight reception end varies.

For example, as shown in FIG. 25, the above-mentioned “signal receptionend” signifies an optical receiver 101 having an optical-electricalconversion function (O/E) to receive dropped optical signal forconverting it into an electric signal, and this also applies to thefollowing description. Moreover, the “signal transmission end” signifiesan optical transmitter 102 having an electrical-optical conversionfunction (E/O) to transmit a transmission signal (electric signal) withadded optical signal with a given wavelength.

For example, as shown in FIG. 19(B), the above-mentioned optical powervariation can depend mainly upon three factors: (1) spectral holeburning (SHB), (2) gain (wavelength) deviation and (3) stimulated Ramanscattering (SRS) effects, which will be described hereinbelow.

(1) SHB

The SHB producing the first factor is a phenomenon arising in theoptical amplifier 200 (300) and shows a characteristic that theshorter-wavelength side light power lowers. That is, for example asshown in FIG. 20, when the optical amplifier 200 (300) amplifies lightwith one wavelength (for example 1538 nm) in the C band (1530 to 1565nm), as a phenomenon, the EDFA gain lowers in the vicinity of thatsignal wavelength (which is referred to as main hole) and the EDFA gainalso lowers in the vicinity of 1530 nm (which is referred to as secondhole).

Moreover, in the C band, the main hole becomes deeper toward theshorter-wavelength side (gain decreasing quantity becomes larger) andboth the main hole and second hole become deeper as the optical signalinput power increases. Since this SHB is averaged in a state wheremulti-wavelength signal is inputted, it does not show a great effect. Onthe other hand, the effect thereof increases with a decrease in numberof input wavelengths. For this reason, in a case in which only a signalwith one wavelength remains due to the occurrence of a trouble betweenthe nodes 100A and 100B as mentioned above, as shown in the column (1)of FIG. 19(B) and in FIG. 21, the gain of the optical amplifier 200(300) decreases as the residual signal shifts toward theshorter-wavelength side, which causes a phenomenon that the outputsignal power also lowers (−ΔP) . That is, the fluctuation of the gaindue to the SHB varies in accordance with the number of signalwavelengths and the allocation thereof. The detail of the SHB is writtenin the non-patent documents 1 to 3 mentioned later.

(2) Gain Deviation

The gain (wavelength) deviation producing the second factor is also aphenomenon occurring in the optical amplifier 200 (300). That is, asmentioned above, the optical amplifier 200 (300) is made to maintain theaverage gain of the signal to a constant value (AGC) and, when awavelength showing a deviation remains, it operates to adjust the gainof that signal to a target gain, which makes a variation (in this case,+ΔP) in output signal power of the residual optical signal, for example,as shown in the column (2) of FIG. 19(B). In other words, the operatingpoint of the optical amplifier 200 (300) varies in accordance with thenumber of signal wavelengths and the arrangement thereof, which causes avariation in gain spectrum.

(3) SRS Effect

The SRS effect producing the third factor is a phenomenon occurring inthe transmission line 400. An optical amplifier utilizing this SRSeffect is a Raman amplifier. For example, as shown in FIG. 22, as acharacteristic, the SRS of a common single-mode fiber has a gain peak onthe lower frequency side by approximately 13 THz from the pumpwavelength (when the pump wavelength is in the vicinity of 1400 nm,longer-wavelength side by approximately 100 nm) and, hence, theselection of the pump wavelength enables the optical signalamplification in an arbitrary wavelength band. However, as shown in FIG.22, the amplification at one-point wavelength is not feasible, and sincethe amplification (gain) characteristic has a range in some degree withrespect to wavelength, the amplification phenomenon arises even in thevicinity of the pump wavelength.

That is, when a WDM optical signal is transmitted through thetransmission line 400, the shorter-wavelength side light power becomespump power, thus amplifying the longer-wavelength side signal. Inconsequence, as shown in FIG. 23, there arises a phenomenon that thesignal power increases toward the longer-wavelength side. Therefore, ina case in which only a signal with one wavelength remains due to theoccurrence of a trouble between the nodes 100A and 100B as mentionedabove, as shown in the column (3) of FIG. 19 and in FIG. 21 (B) , as theresidual signal shifts toward the longer-wavelength side, it is moredifficult to take the power from the shorter-wavelength side, whichcauses the power (gain) reduction (−ΔP) . That is, the SRS effect variesin accordance with the number of signal wavelengths and the allocationthereof.

Thus, when the number of wavelengths of WDM signal transmitted throughthe transmission line 400 varies largely, mainly depending upon thethree f actors of SHB, gain deviation and SRS, the output power of theresidual signal (residual channel) varies. Even if the variationquantity per optical amplifier or transmission line is not very large,in the case of a system in which the optical amplifiers 200 and 300 madeto carry out the AGC are provided in a multi-stage configuration, therearises the accumulation of the output signal power variations (ΔP) ofthe respective channels occurring the respective optical amplifiers 200,300 and transmission lines 400.

In the case of a conventional optical transmission system in which thetransmission distance is short and the number of optical amplifiers tobe disposed in a multi-stage configuration is small, this variation islittle and no problem arises. However, when the number of stages ofoptical amplifiers increases due to the lengthening of the transmissiondistance of the system in the future, for example, as shown in the leftside of FIG. 24, the optical signal power at the signal reception endbecomes out of a reception allowable range, which can producetransmission errors.

It is considered that the accumulation of the power variations ispreventable by carrying out the automatic level control (ALC) at highspeed at the occurrence of variation of the number of wavelengths, forexample, as shown in the right side of FIG. 24. In this case, the ALC isusually made to monitor the output light power of the optical amplifier200 (300) through the use of PD or the like for controlling(feedback-controlling) the pump power to the optical amplifier 200 (300)so that the monitored value reaches a target output signal power Poutwhich is a target output signal power Ptarget [dBm/ch] per channel×thenumber of inputted signal wavelengths.

Thus, for realizing the ALC of the optical amplifier 200 (300), theinformation on the number of inputted signal wavelengths to be inputtedto the optical amplifier 200 (300) becomes necessary. However, in thecase of receiving the information on the number of wavelengths from anoptical service channel (OSC) or a network management system (NMS), ittakes much time and cannot cope with the transient variation, forexample, immediately after the occurrence of a trouble [for example, asindicated by meshing in FIG. 19(A), a variation in a period until theOADM node 100 shows the level compensation function after the occurrenceof variation in the number of wavelengths].

For this reason, there have been proposed some techniques (methods) ofcalculating the number of wavelengths in the interior of a node.

(a) One technique is a method of, on the condition that the opticalpowers of the respective wavelengths to be inputted to an opticalamplifier are even (inputted optical power per wavelength is known inadvance), monitoring the total power of the inputted light to theoptical amplifier to calculate the number of transmission wavelengths bydividing the monitored value by a specified inputted optical power perchannel.

(b) As proposed in the following patent documents 1 and 2, anothertechnique is a method of inputting light, branched in a manner such thata portion of signal inputted to an optical amplifier is used as monitorlight, to a wavelength demultiplexer (DEMUX) to demultiplex it accordingto wavelength for counting the number of transmission wavelengths.Concretely, in the technique disclosed in the patent document 1, theinputted signal to the optical amplifier is monitored according towavelength and the attenuation quantity of a variable optical attenuatorprovided at an output of the optical amplifier is adjusted in accordancewith the monitored value and a variation in the number of wavelengthsfor controlling the output light power collectively. On the other hand,according to the technique disclosed in the patent document 2, in anoptical amplifier in which optical amplification fibers such as EDF areconnected in a multi-stage configuration, the pump power to each opticalamplification fiber and the attenuation quantity of a variable opticalattenuator provided between the stages of the respective opticalamplification fibers are adjusted on the basis of the signal power andthe number of wavelengths, detected from the inputted light to theformer-stage optical amplification fiber, and the light power detectedfrom the output light of the latter-stage optical amplification fiber,thereby controlling the gain of the entire optical amplifier and thegain spectrum.

It is desirable that the above-mentioned transient variation of theoutput power of an optical amplifier is compensated for (undergoes theflattening processing) as quickly as possible (for example, on the orderof microsecond). As candidates for a technique of compensating for suchan output power variation, there are, for example, (c) a configurationin which WDM output signal is demultiplexed according to wavelength andthe optical power of each wavelength is adjusted through the use of avariable optical attenuator (VOA) for each wavelength and thenmultiplexed, (d) a dynamic gain equalizer (DGEQ), and other techniques.The DGEQ is a device designed to perform the loss adjustment for eachwavelength of the WDM signal and capable of compensating for the gaindeviation.

In addition, as a technique about the gain equalization, there aretechniques disclosed in the following patent documents 3 and 4. Thetechnique disclosed in the patent document uses an optical circulator,an optical reflector, a variable optical attenuator and a WDM couplerfor carrying out the gain equalization according to a plurality ofsignals (wavelengths) demultiplexed by the WDM coupler. Moreover, thetechnique disclosed in the patent document 4 is related to variable gainflattening unit including a plurality of long-period gratingsarrangement and an adjustment unit (piezo converter and piezo controlcircuit) for adjusting the attenuation factor for each grating.

[Patent Document 1] Japanese Patent Laid-Open No. 2001-168841

[Patent Document 2] Japanese Patent Laid-Open No. 2003-258348

[Patent Document 3] Japanese Patent Laid-Open No. HEI 10-173597

[Patent Document 4] Japanese Patent Laid-Open No. HEI 11-337750

[Non-Patent Document 1] Masato NISHIHARA, et. al., “Characterization andnew numerical model of spectral hole burning in broadband erbium-dopedfiber amplifier”, 2003 Optical Society of America.

[Non-Patent Document 2] Masato NISHIHARA, et. al., “Impact of spectralhole burning in multi-channel amplification of EDFA”, 2004 OpticalSociety of America.

[Non-Patent Document 3] Maxim Bolshtyansky, “Spectral Hole Burning inErbium-Doped Fiber Amplifiers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.21, NO. 4, APRIL 2003.

However, with respect to the technique (method) of calculating thenumber of wavelengths in a node, there is a problem which arises withthe method described above in (a) in that, when the power of eachchannel to be inputted to an optical amplifier varies, there is apossibility of calculating the number of wavelengths in error. Moreover,the method described above in (b) creates a problem of high cost andsize increase. In the case of the technique disclosed in the patentdocument 1, since the loss is large in the wavelength demultiplexer(DEMUX), there is a need to increase the input branch ratio, whichcauses the deterioration of the noise characteristic (NF) of an opticalamplifier, and in the case of the technique disclosed in the patentdocument 2, there is a problem in that the high-speed response becomesdifficult.

Moreover, with respect to the equalization technology, in the case ofthe technique described above in (c) and the technique disclosed in theaforesaid patent document, since the received WDM signal isdemultiplexed according to wavelength and the adjustment of the opticalpower is made according to wavelength through the use of the variableoptical attenuator for each wavelength, the apparatus scale becomeslarger to increase the cost. In particular, if a high-speed operatingVOA is used for obtaining a high-speed response characteristic, this VOAis expensive and, when the VOAs corresponding in number to wavelengthsare prepared, a further increase in cost occurs.

Still moreover, the dynamic gain equalizer described above in (d)creates problems in that the response speed is as relatively low asapproximately 30 ms and the cost thereof stands at several millions yenand the adding loss is large (approximately 5 dB). Accordingly, theintroduction into the system is impractical. Yet moreover, in the caseof the technique disclosed in the aforesaid patent document 4, although,in a manner such that the piezo control circuit controls the piezoconverter to change the pressure to be applied to the gratings, thecharacteristics of a plurality of grating can individually be changed soas to vary the attenuation factor of light passing through the gratings,the response speed is low because the change of pressure to the gratingsfalls under physical control.

SUMMARY OF THE INVENTION

The present invention has been developed in consideration of theabove-mentioned problems, and it is therefore an object of the inventionto suppress the transient variation of a signal power level due to theabove-mentioned SHB or SRS with a simple configuration withoutdeteriorating the noise characteristic for allowing optical amplifiersto be further disposed in a multi-stage system, thereby enablinglengthening the transmission distance of a transmission system includingan optical add/drop unit.

Another object is to compensate easily for the deviation in wavelengthcharacteristic in a signal wavelength band at a high speed and at a lowcost.

For these purposes, the present invention is characterized by employinga control apparatus and method for the following optical amplifier, anindividual band gain equalizer, a wavelength multiplexing transmissionapparatus, an optical amplifier at a wavelength multiplexingtransmission system using the same equalizer.

(1) A control apparatus for an optical amplifier according to thepresent invention comprises an automatic gain control unit forcontrolling a gain of the optical amplifier to a constant value based onpower of incoming light and out going of the optical amplifier and atarget gain, an individual band incoming light monitoring unit fordividing a signal wavelength band of the incoming light into at least afirst band and second band: said first band has tendency of decreasingpower of the outgoing light at an decrease in the number of signalwavelengths and said second band, is a signal wavelength band other thansaid first band and, including a gain deviation band in which power ofthe outgoing light varies mainly due to a wavelength deviation of thegain control by said automatic gain control unit, and for monitoringpower of the incoming light to said optical amplifier in each of thefirst and second bands, an individual band signal wavelength numbercalculation unit for obtaining the number of signal wavelengths in eachof the first and second bands based on a result of the monitoring by theindividual band incoming light monitoring unit, and a target gaincorrection unit for correcting the target gain to be used by theautomatic gain control unit based on a result of the calculation by theindividual band signal wavelength number calculation unit at a variationof the number of signal wavelengths.

(2) In this case, preferably, the aforesaid individual band incominglight monitoring unit divides the signal wavelength band into threebands of an SHB band, said gain deviation band and SRS band, said SHBband being under dominance of a spectral hole burning (SHB) effect asthe band, the gain deviation band and an SRS band under a stimulatedRaman scattering (SRS) effect as said first band, and said SRS bandbeing under dominance of a stimulated Raman scattering (SRS) effectoccurring in an output transmission line of said optical amplifier asanother band of said first band, and monitors power of the incominglight in each of said three bands.

(3) More concretely, the signal wavelength band is a C band, and the SHBband is from 1530 nm to 1540 nm, the SRS band is from 1555 nm to 1565nm, and the gain deviation band is a band interposed between the SHBband and the SRS band.

(4) In addition, preferably, individual band signal wavelength numbercalculation unit divides values of power monitored for the individualdivided bands by said individual band incoming light monitoring unit bya design value of power of signal light per signal wavelength, anddetermines a value closest to the nth power of 2 from the resultantvalue obtained by the division as the number of signal wavelengths ineach of the divided bands.

(5) Preferably, a threshold to be used for the determination of theaforesaid number of signal wavelengths is set on the basis of acharacteristic of output light power variation in each of the aforesaiddivided bands.

(6) In this case, threshold for said first band is set to be smallerthan a threshold for said second band including said gain deviationband.

(7) Moreover, preferably, when recognizing, based on a result of thecalculation by said individual band signal wavelength number calculationunit, that the number of signal wavelengths in said first band becomessmaller than a predetermined value.

(8) Still moreover, preferably, when recognizing, based on a result ofthe calculation by said individual band signal wavelength numbercalculation unit, that the number of signal wavelengths in said gaindeviation band becomes smaller than a predetermined value, in a statewhere the remaining number of signal wavelengths in said first bandexceeds a predetermined value.

(9) Yet moreover, preferably, target gain correction unit updates saidtarget gain based on a result of the calculation by said individual bandsignal wavelength number calculation unit until a predetermined periodof time elapses from the occurrence of a variation of the number ofsignal wavelengths and, after the elapse of said predeterminded periodof time, brings said target gain gradually closer to a specified gainvalue.

(10) Furthermore, in accordance with the present invention a controlmethod for an optical amplifier having an automatic gain controlfunction to control a gain of the optical amplifier to a constant valuebased on power of incoming light and outgoing light of the opticalamplifier and a target gain, comprises the steps of dividing a signalwavelength band of the incoming light into at least a first band andsecond band: said first band has tendency of decreasing power of theoutgoing light at a decrease in the number of signal wavelengths andsaid second band, is a signal wavelength band other than said band,including a gain deviation band in which power of the outgoing lightvaries mainly due to a wavelength deviation of the gain control, andmonitoring power of the incoming light in each of the divided bands,obtaining the number of signal wavelengths in each of the divided bandsbased on a result of the monitoring, and correcting said target gain tobe used for the automatic gain control based on the number of signalwavelengths in each of the divided bands at a variation of the number ofsignal wavelengths.

(11) Still furthermore, An optical transmission apparatus comprising:

an automatic gain control unit for controlling a gain of an opticalamplifier to a constant value based on power of incoming light andoutgoing light of said optical amplifier and a target gain, anindividual band incoming light monitoring unit for dividing a signalwavelength band of the incoming light into at least a first band andsecond band: said first band has tendency of decreasing power of theoutgoing light per channel at a decrease in the number of signalwavelengths or increasing power of the outgoing light per channel at anincrease in the number of signal wavelengths and asid second band, is asignal wavelength band other than said band, including a gain deviationband in which power of the outgoing light per channel varies mainly dueto a wavelength deviation of the gain control by said automatic gaincontrol unit, and for monitoring power of the incoming light in each ofthe divided bands, an individual band signal wavelength numbercalculation unit for obtaining the number of signal wavelengths in eachof the divided bands based on a result of the monitoring by saidindividual band incoming light monitoring unit, and a target gaincorrection unit for correcting said target gain to be used by saidautomatic gain control unit based on a result of the calculation by saidindividual band signal wavelength number calculation unit at a variationof the number of signal wavelengths.

(12) In this case, preferably, the a fore said individual band inputlight monitoring unit divides said signal wavelength band into threebands: an SHB band, said gain deviation band and an SRS band, said SHBband being under dominance of a spectral hole burning (SHB) effect assaid first band and said SRS band being under dominance of a stimulatedRaman scattering (SRS) effect occurring in an output transmission lineof said optical amplifier as another band of said first band , andmonitors power of the incoming light in each of said three bands.

(13) Preferably, the SHB band is from 1530 nm to 1540 nm, said SRS bandis from 1555 nm to 1565 nm, and said gain deviation band is a bandinterposed between said SHB band and said SRS band.

(14) Moreover, preferably, the individual band signal wavelength numbercalculation unit divides values of power monitored for the individualdivided bands by said individual band incoming light monitoring unit bya design value of power of signal light per signal wavelength , anddetermines a value closest to the nth power of 2 from the resultantvalue obtained by the division as the number of signal wavelengths ineach of the divided bands.

(15) Still moreover, preferably, a threshold to be used for thedetermination of the aforesaid number of signal wavelengths is set onthe basis of a characteristic of output light power variation perchannel in each of the aforesaid divided bands.

(16) Yet moreover, preferably, the threshold for the first band is setto be smaller than a threshold for the second band including theaforesaid gain deviation band.

(17) In addition, it is also appropriate that, when recognizing, basedon a result of the calculation by said individual band signal wavelengthnumber calculation unit, that the number of signal wavelengths in saidfirst band becomes smaller than a predetermined value.

(18) Still additionally, it is also appropriate that, when recognizing,based on a result of the calculation by said individual band signalwavelength number calculation unit, that the number of signalwavelengths in said gain deviation band becomes smaller than apredetermined value, in a state where the remaining number of signalwavelengths in said first band exceeds a predetermined value.

(19) Yet additionally, it is also appropriate that the target gaincorrection unit updates the target gain on the basis of the result ofthe calculation by the band-by-band signal wavelength number calculationunit until a given period of time elapses from the occurrence of avariation of the number of signal wavelengths and, after the elapse ofthe given period of time, brings the target gain gradually closer to aspecified gain value.

(20) Furthermore, An optical amplifier having an automatic gain controlfunction to control a gain to a constant value based on power ofincoming light and outgoing light and a target gain, wherein a signalwavelength band of an incoming light into at least a first band andsecond band: said first band has tendency of decreasing power of theoutgoing light per channel at a decrease in the number of signalwavelengths or increasing power of the outgoing light per channel at anincrease in the number of signal wavelengths and said second band, is asignal wavelength band other than said band, including a gain deviationband in which power of the outgoing light varies mainly due to awavelength deviation of the gain control for monitoring power of theincoming light in each of the divided bands, obtaining the number ofsignal wavelengths in each of the divided bands based on a result of themonitoring, and correcting said target gain to be used for the automaticgain control based on the number of signal wavelengths in each of thedivided bands at a variation of the number of signal wavelengths.

(21) Still furthermore, a band unit gain equalizer according to thepresent invention comprises a band division means for dividing a signalwavelength band of an incoming wavelength multiplexed light into atleast a first band and second band: said first band has tendency ofdecreasing power of outgoing light of an optical amplifier per channelat a decrease in the number of signal wavelengths or increasing power ofthe outgoing light of said optical amplifier per channel at an increasein the number of signal wavelengths and said second band, is a signalwavelength band other than said first band, including a gain deviationband in which power of the outgoing light per channel varies mainly dueto a wavelength deviation of automatic gain control in the opticalamplifier, and an adjustment means for adjusting the power of theoutgoing light for the individual divided bands devided by the banddivision means.

(22) In this case, it is also appropriate that the band division meansdivides said signal wavelength band into three bands: an SHB band, saidgain deviation band and an SRS band, said SHB band being under dominanceof a spectral hole burning (SHB) effect as said first band, and said SRSband being under dominance of a stimulated Raman scattering (SRS) effectoccurring in an output transmission line of said optical amplifier asanother band of said first band.

(23) In addition, it is also appropriate that the adjustment meansincludes a variable optical attenuator for the SHB band, a variableoptical attenuator for the gain deviation band and a variable opticalattenuator for the SRS band, and the band division means includes a gaindeviation band reflection device for reflecting light in the gaindeviation band, a first optical circulator for leading the incomingwavelength multiplexed light to the gain deviation band reflectiondevice and leading reflected light from the gain deviation bandreflection device to the variable optical attenuator for the gaindeviation band, a band separation device for dividing light passingthrough the gain deviation band reflection device into lights in the SHBband and the SRS band to lead the lights to the variable opticalattenuators for the SHB band and the SRS band, a band coupling devicefor coupling output lights of the SHB band and SRS band variable opticalattenuators, and a second optical circulator provided on an output sideof the gain deviation band variable optical attenuator for leadingoutput light from the band coupling device to the gain deviation bandvariable optical attenuator side and for leading light inputted from thegain deviation band variable optical attenuator side to an output port,with an SHB band reflection device, provided between said gain deviationband variable optical attenuator and said second optical circulator, forreflecting, of light led to said gain deviation band variable opticalattenuator side, light in said SHB band and an SRS band reflectiondevice, provided between said gain deviation band variable opticalattenuator and said second optical circulator, for reflecting, of lightled to said gain deviation band variable optical attenuator side, lightin said SRS band.

(24) Preferably, each of the variable optical attenuators is ahigh-speed variable optical attenuator having a response speed on theorder of microsecond.

(25) In addition, A wavelength multiplexing transmission apparatus usingan individual band gain equalizer according to the present inventioncomprises the individual band gain equalizer described above in the item(21) and control means for monitoring power of input light or outputlight of the individual band gain equalizer for each of the dividedbands to control the output light power adjustment for the individualdivided bands in the adjustment means on the basis of a result of themonitoring so that the output light power of each of the divided bandsbecomes a predetermined target value.

(26) In this case, it is also appropriate that the control meansincludes an individual band monitoring unit for monitoring power ofinput light or output light of said individual band gain equalizer foreach of the divided bands, a storage unit for storing the aforesaidtarget value in advance, a difference detection unit for detecting adifference between a result of the monitoring by the individual bandmonitoring unit and the target value in the storage unit for each of thedivided bands, and a gain equalization control unit for controlling theadjustment means so that the difference detected by the differencedetection unit for each of the divided bands reaches a minimum.

(27) Moreover, it is also appropriate that the wavelength multiplexingtransmission apparatus is arranged as a wavelength add/drop apparatushaving an add/drop unit made to add/drop light with a wavelength formingat least a portion of the inputted wavelength multiplexed light, and theband individual gain equalizer is provided at the former stage or latterstage of the add/drop unit.

(28) Furthermore, a wavelength multiplexing transmission system using aindividual band gain equalizer according to the present inventioncomprises a wavelength multiplexing transmission apparatus describedabove in the item (25).

(29) Still furthermore, An optical amplifier using an individual bandgain equalizer according to the present invention comprises anamplification medium for an amplification medium for amplifying incomingwavelength multiplexed light, an automatic gain control unit forcarrying out automatic gain control based on power of incoming light andoutgoing light of said amplification medium and a target gain, anindividual band gain equalizer for dividing a signal wavelength band ofan outgoing light of said amplification medium into at least a firstband and second band: said first band has tendency of decreasing powerof the outgoing light per channel at a decrease in the number of signalwavelengths or increasing power of the outgoing light per channel at anincrease in the number of signal wavelengths and said second band, is asignal wavelength band other than said first band, including a gaindeviation band in which power of the outgoing light varies mainly due toa wavelength deviation of the automatic gain control so as to adjust thepower of the outgoing light for the individual divided bands, andcontrol means for monitoring the power of the outgoing light of saidamplification medium for each of the divided bands to control saidoutput light power adjustment of said band unit gain equalizer for eachof the divided bands based on a result of the monitoring so that thepower of the outgoing light of each of the divided bands becomes apredetermined target value.

(30) Yet furthermore, a wavelength multiplexing transmission systemaccording to the present invention comprises the optical amplifierdescribed above in the item (29).

According to the present invention, the target gain is corrected on thebasis of the number of signal wavelengths obtained by monitoring inputlight power for each of the divided bands and, hence, even if the numberof signal wavelengths varies suddenly due to a trouble or the like sothat the input light power to the optical amplifier varies suddenly, itis possible to suppress the variation (transient variation) of theoutput signal power (residual signal power) to a minimum, thussuppressing the accumulation of the transient output power variation.Therefore, the light power at the signal reception end lies in thereception power allowable range of a receiver, and the considerableoptical S/N ratio deterioration at the signal reception end due to theaccumulation of the light power reduction is improvable. In addition,since there is no need to perform the demultiplexing in units ofwavelength for obtaining the number of signal wavelengths unlike theconventional technique, the degradation of the noise characteristic (NF)of the optical amplifier is suppressible.

In particular, when the signal wavelength band is divided into threebands of the SHB, the gain deviation band and the SRS band and the inputlight power of each of the bands is monitored for obtaining the numberof signal wavelengths in each band, the gain correction can be made inaccordance with the tendency of output light power variation in eachband, thereby providing greater transient variation suppression effectson the output signal power.

Moreover, according to the present invention, since the output lightpower can be adjusted independently for each of the divided bands, thecompensation for the gain deviation (output light power deviation)arising in the signal wavelength band can be made in unit of the dividedband instead of unit of wavelength. This can easily realize a gainequalizer at a low cost.

When the output power adjustment in units of the divided bands iscarried out through the use of a high-speed variable attenuator having aresponse speed on the order of microsecond, since the compensation forthe output power variation occurring in the aforesaid signal wavelengthband becomes feasible, even in a case in which the signal light state(number of wavelengths and allocation of wavelengths) varies (increasesor decreases) suddenly so that difficulty is experienced in achieve thecompensation in a state where an existing node such as optical add/dropapparatus follows the variation, the accumulation of the output powervariation does not occur and, even if the transmission is made in amulti-stage configuration, the occurrence of transmission error ispreventable at the reception end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a portion of a WDMoptical transmission system according to a first embodiment of thepresent invention;

FIG. 2 is a block diagram showing a configuration of an opticalamplifier (post amplifier) according to this embodiment;

FIG. 3 is an illustration of an example of division of a signalwavelength band according to this embodiment;

FIG. 4 is an illustration useful for explaining a band showing a strongSHB effect;

FIG. 5 is an illustration useful for explaining a band showing a strongSRS effect;

FIGS. 6(A) to 6(C) are illustrations of the relationship between thenumbers of signal wavelengths to be obtained from inputted lightmonitored values in divided bands for explaining an operation of anindividual band signal wavelength number calculation unit shown in FIG.2;

FIG. 7 is an illustration of examples of input signal light power in asignal wavelength band, average input signal light power in the signalwavelength band and a specified input signal light power for explainingan operation of a target gain calculation unit shown in FIG. 2;

FIG. 8 is a flow chart useful for explaining a gain control method forthe optical amplifier shown in FIG. 2;

FIG. 9 is an illustration of a control image for target gain setting forexplaining the gain control method for the optical amplifier shown inFIG. 2;

FIG. 10 is an illustration of one example of a signal level variationwith respect to the number of transit EDFAs for explaining the effectsof the gain control method for the optical amplifier shown in FIG. 2 incomparison with a conventional technique;

FIG. 11 is an illustration of one example of optical SN ratio forexplaining the effects of the gain control method for the opticalamplifier shown in FIG. 2 in comparison with a conventional technique;

FIG. 12 is an illustration of one example of a target gain setting tableto be used in a modification of the gain control method according tothis embodiment;

FIG. 13 is a flow chart useful for explaining a modification of the gaincontrol method according to this embodiment;

FIG. 14 is a block diagram showing a modification of input monitoringmeans shown in FIG. 2;

FIG. 15 is a block diagram showing a modification of input monitoringmeans shown in FIG. 2;

FIG. 16 is a block diagram showing an example of a configuration of ametro core system;

FIG. 17 is a block diagram showing an example of a configuration of aconventional AGC amplifier;

FIGS. 18(A) and 18(B) are illustrations for explaining an operation atthe occurrence of a trouble in the system shown in FIG. 16;

FIGS. 19(A) and 19(B) are illustrations for explaining an output lightpower variation at a variation of the number of wavelengths due to theoccurrence of a trouble in the system shown in FIG. 16;

FIG. 20 is an illustration of one example of a wavelength dependencygain variation quantity due to SHB;

FIG. 21(A) is an illustration of one example of a gain variationquantity due to SHB with respect to wavelengths;

FIG. 21(B) is an illustration of one example of a gain variationquantity due to SRS with respect to wavelengths;

FIG. 22 is an illustration of a gain spectrum of Raman amplificationband;

FIG. 23 is an illustration for explaining a Raman effect betweensignals;

FIG. 24 is an illustration for explaining problems of a conventionaltechnique;

FIG. 25 is a block diagram for explaining a signal reception end;

FIG. 26 is a block diagram showing a configuration of a gainequalization device (individual band gain equalizer) according to asecond embodiment of the present invention;

FIG. 27 is an illustration for explaining an operation of a gainequalization device shown in FIG. 26 and shows an optical path;

FIG. 28 is a block diagram showing a configuration of an OADM nodeemploying the gain equalization device shown in FIG. 26;

FIG. 29 is a block diagram showing a different configuration of an OADMnode employing the gain equalization device shown in FIG. 26;

FIGS. 30(A) and 30(B) are illustrations of images of variation of outputpower in an SHB band, a gain deviation band and an SRS band by gainequalization control in the OADM node shown in FIG. 28 or 29;

FIG. 31 is a flow chart useful for explaining the gain equalizationcontrol in the OADM node shown in FIG. 28 or 29;

FIG. 32(A) to 32(F) are illustrations of states of power variation in asignal wavelength band in the case of a steady condition and theoccurrence of variation of a signal light state in the OADM node shownin FIG. 28 or 29;

FIG. 33 is an illustration for explaining the effects of the OADM nodeshown in FIG. 28 or 29 in comparison with a conventional configuration;

FIG. 34 is a block diagram showing a configuration of a WDM transmissionsystem for explaining a WDM transmission system adjustment method usingthe gain equalization device shown in FIG. 26;

FIGS. 35(A) and 35(B) are illustrations of an image of variation ofoutput power in an SHB band, a gain deviation band and an SRS band bythe WDM transmission system adjustment method shown in FIG. 34;

FIG. 36 is a block diagram showing a configuration of an opticalamplifier using the gain equalizer shown in FIG. 26;

FIG. 37 is a flow chart useful for explaining gain equalization controlin the optical amplifier shown in FIG. 36;

FIGS. 38(A) to 38(F) are illustrations of states of power variation in asignal wavelength band in the case of a steady condition and theoccurrence of variation of a signal light state in the optical amplifiershown in FIG. 36

FIG. 39 is a block diagram showing a configuration of a WDM transmissionsystem using the optical amplifier shown in FIG. 36;

FIG. 40 is a block diagram showing a first modification of the gainequalization device shown in FIG. 26;

FIG. 41 is an illustration of a transmission characteristic of a firstedge filter in the gain equalization device shown in FIG. 40;

FIG. 42 is an illustration of a transmission characteristic of a secondedge filter in the gain equalization device shown in FIG. 40;

FIG. 43 is an illustration of a transmission characteristic of the gainequalization device shown in FIG. 40;

FIG. 44 is a block diagram showing a second modification of the gainequalization device shown in FIG. 26;

FIG. 45 is an illustration of a transmission characteristic of a firstgrating of the gain equalization device shown in FIG. 43;

FIG. 46 is an illustration of a transmission characteristic of a secondgrating of the gain equalization device shown in FIG. 43;

FIG. 47 is an illustration of a transmission characteristic of the gainequalization device shown in FIG. 43;

FIG. 48 is a block diagram showing a third modification of the gainequalization device shown in FIG. 26;

FIG. 49 is an illustration of a signal wavelength arrangement at eachportion for explaining an operation of the gain equalization deviceshown in FIG. 48;

FIG. 50 is a block diagram showing a fourth modification of the gainequalization device shown in FIG. 26; and

FIG. 51 is an illustration of a signal wavelength arrangement at eachportion for explaining an operation of the gain equalization deviceshown in FIG. 50.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [A] Description of FirstEmbodiment

FIG. 1 is a block diagram showing a configuration of a portion of a WDMoptical transmission system according to a first embodiment of thepresent invention. The configuration shown in FIG. 1 corresponds to onespan (from one OADM node 100 to an OADM node 100 adjacent thereto) ofthe system described above with reference to FIG. 16, and between theOADM nodes 100, there are provided an optical amplifier (post amplifier)1, a transmission line 2, an optical amplifier (first preamplifier) 3, adispersion compensation module (DCM) 4 and an optical amplifier (secondpreamplifier) 5.

In this configuration, the post amplifier 1 is for amplifying WDM signalfrom the former-stage OADM node 100 up to a predetermined signal powerlevel, and an ALC according to the present invention is applied theretowhen a considerable variation in number of wavelengths occurs. Moreover,the preamplifier 3 is for amplifying WDM signal, transmitted in a statewhere a loss exists in the transmission line 2, to a predeterminedsignal power level. The DCM 4 is for compensating for the chromaticdispersion the WDM signal receives from the transmission line 2, and thepreamplifier 5 is for amplifying inputted WDM signal up to apredetermined signal power level for compensating for the loss in theDCM 4.

With this configuration, the WDM signal transmitted from one OADM node100 is transmitted to the next-stage OADM node 100 while being amplifiedby each of the aforesaid amplifiers 2, 3 and 5 and in a state where thecompensation for the chromatic dispersion is made in the DCM 4. However,in this embodiment, the post amplifier 1 is made to easily and quicklymeasure the actual number of wavelengths included in the inputted WDMsignal according to a special method for carrying out the automaticlevel control (ALC) (output power constant control), thereby reducing anoutput signal power variation (which will herein after be equallyreferred to simply as “power variation”), which occurs mainly due tothree factors (SHB, gain deviation, SRS effect) when the number ofwavelengths in the WDM signal varies suddenly, to a minimum.

That is, as shown in FIG. 3, with respect to a signal wavelength band(which will herein after be referred to equally as a “signal band”) 40of the inputted WDM signal, the post amplifier 1 according to thepresent invention monitors inputted light power for each of three bandsof a wavelength band (hereinafter referred to as an “SHB band”) 41 inwhich the power variation due to SHB dominantly occurs (the SHB effectdominantly appears), a wavelength band (hereinafter referred to as a“gain deviation band) 42 in which the power variation due to the gaindeviation dominantly occurs (the gain deviation effect greatly appears)and a wavelength band (hereinafter referred to as an “SRS band”) 43 inwhich the power variation due to the SRS effect greatly occurs (the SRSeffect greatly appears) for estimating (approximate) the number ofwavelengths included in each of the bands 41, 42 and 43 on the basis ofeach of the monitored values so as to obtain the number of wavelengthsincluded in the signal wavelength band 40. Thus, at a variation innumber of wavelengths, unlike the conventional technique, withoutreceiving information on the number of wavelengths from the OSC or NMSand without carrying out the power monitoring for each wavelengthincluded in the signal band 40, the number of signal wavelengths at thevariation in number of wavelengths is easily and quickly obtained toadjust (correct) the AGC target gain, thereby carrying out the ALC.

Each of the aforesaid SHB band 41 and the SRS band 43 correspond to aband in which the output signal power of the amplifier (opticalamplifier) 1 per channel tends to decrease or in which the output signalpower of the amplifier 1 per channel tends to increase at an increase inthe number of signal wavelengths. Moreover, as shown in FIG. 4, the SHBeffect (depth of a main hole) greatly appears when the wavelength isbelow 1540 nm and is generally constant when the wavelength is above1545 nm and, hence, the SHB band 41 can be set to be, for example, 1530to 1540 nm. Still moreover, as shown in FIG. 5, the SRS effect has aninclination having a generally linear deviation (dB) with respect towavelengths at input of 40 waves in the C-band and, hence, the SRS band43 can be set to be, for example, 1555 to 1565 nm.

For realizing the above-mentioned ALC, for example, as shown in FIG. 2,the amplifier 1 according to this embodiment includes an EDF 11 and, aselements constituting a control unit for the amplifier 1, on the inputside of the EDF 11, optical couplers 12, 13 and 14 serving as input sideoptical branch means, band-pass filters 15-1, 15-2 and photodiodes (PD)17-1, 17-2, 17-4 serving as light-receiving elements and, on the outputside of the EDF 11, an optical coupler 18 serving as output side opticalbranch means and a PD 19 serving as a light-receiving element and,further, includes an automatic gain control unit (gain constant control)20, a band-by-band (individual band) signal wavelength numbercalculation unit 21, a target gain calculation unit 22 and a storageunit (database) 23.

In this configuration, the optical coupler 12 is for power-separating aportion of the inputted signal light (WDM signal) as inputted monitorlight to output it to the optical coupler 13, and the optical coupler 13is for branching the inputted monitor light into two to input them tothe optical coupler 14 and the PD 17-4 for the signal band 40. Moreover,the optical coupler (WDM coupler: 3 dB coupler) 14 divides the band ofthe inputted monitor light from the optical coupler 13 into two in thevicinity of a central wavelength of the signal band 40 to input them tothe band-pass filters 15-1 and 15-2, and the band-pass filter 15-1permits, of the inputted monitor light from the optical coupler 13, onlya signal light component in the SHB band 41 to pass and, likewise, theband-pass filter 15-2 permits only a signal light component in the SRSband 43 to pass, and they have, for example, a band passage width ofapproximately 7 nm to 10 nm.

The PD 17-1 receives the signal light of the SHB band 41 passing throughthe band-pass filter 15-1 to output an electric signal with a levelcorresponding to the light reception quantity as a monitored value ofthe signal light power of the SHB band 41 to the band-by-band signalwavelength number calculation unit 21. Likewise, the PD 17-2 receivesthe signal light of the SRS band 43 passing through the band-pass filter15-2 to output an electric signal with a level corresponding to thelight reception quantity as a monitored value of the signal light powerof the SRS band 43 to the band-by-band signal wavelength numbercalculation unit 21. Moreover, the PD 17-4 receives the signal light ofthe signal band 40 from the optical coupler 12 to output an electricsignal with a level corresponding to the light reception quantity as amonitored value of the signal light power (total inputted light power)of the signal band 40 to the automatic gain control unit 20.

That is, the optical amplifier 1 according to the present inventionmonitors the inputted signal light powers of the SHB band 41 and the SRSband 43 through the use of the PD 17-1 and the PD 17-2, and monitors thesignal light power (total inputted light power) of the signal band 40through the use of the PD 17-4. In other words, the above-mentionedoptical couplers 13, 14, the band-pass filters 15-1, 15-2 and the PDs17-1, 17-2 function as a band-by-band (individual band) input lightmonitoring unit 10 to separately monitor the inputted signal light powerof each of the SHB band 41 and the SRS band 43. It is possible to obtainthe signal light power of a band other than the SHB band 41 and the SRSband 43, i.e., the gain deviation band 42, by subtracting the respectivemonitored values on the SHB band 41 and the SRS band 43 from themonitored value of the PD 17-4. Naturally, it is also acceptable toprovide a PD for each of the bands 41, 42 and 43 for monitoring theinputted light power thereof, as will be described later with referenceto FIGS. 14 and 15.

The optical coupler 18 is for separating a portion of the output lightof the EDF 11 as a monitor light to input it to a PD 19, and the PD 19receives the outputted monitor light from the optical coupler 18 tooutput an electric signal with a level corresponding to the lightreception quantity to the automatic gain control unit 20.

The band-by-band signal wavelength number calculation unit 21 estimates(approximates) the number of signal light wavelengths included in eachof the SHB band 41, the gain deviation band 42 and the SRS band 43 onthe basis of the monitored value of the signal light power of each ofthe bands 41, 42 and 43. For example, the monitor values of the SHB band41 and the SRS band 43 and the signal light power obtained through thecalculation on the gain deviation band 42 interposed between these bands41 and 43 are divided by a signal light power designed for onewavelength (channel), and of these values, a value closest to the nthpower of 2 (1, 2, 4, 8, 16, . . . ) can be selected and determined asthe number of signal light wavelengths.

In more detail, for example, as shown in FIG. 6(A), thresholds P₁, P₂,P₃, P₄, . . . are set with respect to the monitored value P on the SHBband 41, and the number of signal wavelengths of the SHB band 41 can beobtained (approximated) such that, if P≦P₁, the number of signalwavelengths=2⁰=1, if P₁<P≦P₂, the number of signal wavelengths=2¹=2, ifP₂<P≦P₃, the number of signal wavelengths=2²=4, and if P₃<P≦P₄, thenumber of signal wavelengths=2²=8.

In like manner, as shown in FIG. 6(B), thresholds P₁′, P₂′, P₃′, P₄′,P₅′, . . . are set with respect to the monitored value P′ on the gaindeviation band 42, and the number of signal wavelengths of the gaindeviation band 42 can be obtained such that, if P′≦P₁′, the number ofsignal wavelengths=2⁰=1, if P₁′<P′≦P₂′, the number of signalwavelengths=2¹=2, if P₂′<P′≦P₃′, the number of signal wavelengths=2²=4,if P₃′<P′≦P₄′, the number of signal wavelengths=2³=8, and if P₄′<P′≦P₅′,the number of signal wavelengths=2⁴=16. Moreover, as shown in FIG. 6(C),thresholds P₁″, P₂″, P₃″, P₄″, . . . are set with respect to themonitored value P″ on the SRS band 43, and the number of signalwavelengths of the SRS band 43 can be obtained such that, if P″≦P₁″, thenumber of signal wavelengths=2⁰=1, if P₁″<P″≦P₂″, the number of signalwavelengths=2¹=2, if P₂″<P″≦P₃″, the number of signal wavelengths=2²=4,and if P₃″<P″≦P₄″, the number of signal wavelengths=2³=8.

However, for the calculation of the number of signal wavelengths asdescribed above, there is a case in which it is required to consider theASE power applied from the upstream side. In this case, it is acceptablethat the ASE power of the upstream side optical amplifier is receivedthrough the use of a monitor signal or the like and the ASE power isdivided according to the band widths of the bands 41, 42 and 43 andsubtracted from the monitored values of the bands 41, 42 and 43 so thatthe number of signal wavelengths can be calculated on the basis of thesevalues.

In addition, it is preferable that the aforesaid thresholds are set onthe basis of the features of the power variations of the respectivebands 41, 42 and 43. Accordingly, it is preferable that they aredifferent among the respective bands 41, 42 and 43. For example, in thecase of the SHB band 41, since the signal light power tends to decreasedue to the influence of SHB, it is preferable that the thresholdtherefor is set to be smaller than the threshold for the gain deviationband 42. Moreover, in the case of the SRS band 43, since the influenceof SRS disappears and the signal light power tends to decrease,likewise, it is preferable that the threshold therefor is set to besmaller than the threshold for the gain deviation band 42.

The storage unit 23 is made to store the aforesaid thresholds for thebands, the a fore said ASE power information, the design value ofinputted signal power per wavelength, the signal (wavelength)allocation-to-pump current information needed for the feed forwardcontrol, and the signal (wavelength) allocation-to-target gaininformation needed for the feedback control.

For example, as shown in FIG. 7, the target gain calculation unit(target gain correction unit) 22 calculates average inputted signalpower per wavelength for each of the bands 41, 42 and 43 from theinformation on the number of wavelengths obtained by the aforesaidband-by-band signal wavelength number calculation unit 21 and obtainsthe average input light signal power of the entire signal band 40 on thebasis of a result of the calculation to obtain a correction quantity(gain correction quantity) on the target gain needed for thecancellation of a difference from a specified inputted signal power, andthe automatic gain control unit 20 adjusts (corrects) the gain of theEDF 11 in accordance with this gain correction quantity, thussuppressing the variation in output signal power level. As will bementioned later, this target gain calculation unit 22 can also obtain adifference from the specified target output signal power per wavelengthas the aforesaid gain correction quantity after the calculation of theaverage inputted signal power per wavelength. Moreover, it is alsoappropriate that, instead of the gain correction quantity, a new targetgain reflecting this gain correction quantity is set with respect to theautomatic gain control unit 20.

A detailed description will be given hereinbelow of a gain controlmethod for the optical amplifier 1 according to this embodiment thusconfigured.

For example, as shown in FIG. 8, in the steady state (state in which thevariation of the number of signal wavelengths is smaller than aspecified value), the automatic gain control unit 20 carries out theautomatic gain control on the basis of the monitored values of theinput/output light power of the EDF 11 obtained by the PD 17-4 and thePD 19 so that the gain of the EDF 11 reaches a predetermined gain value(steps S1, S2 and, through Yes route of step S3, step S4). At this time,as mentioned above, the band-by-band signal wavelength numbercalculation unit 21 of the gain adjustment unit 30 counts the number ofsignal wavelengths in each of the bands 41, 42 and 43 so as to monitorthe variation in the number of signal wavelengths (steps S2, S3).

In this state, for example, assuming that more-than-half (39 channels of40 channels) of all the number of signal wavelengths are not inputtedbecause of the occurrence of a trouble or the like so that the totalinputted light power to the optical amplifier 1 (monitored by the PD17-4) considerably decreases (see an arrow 51 and reference mark A inFIG. 9), the target gain calculation unit 22 calculates the averageinputted signal power per wavelength on the basis of the information onthe number of wavelengths obtained by the band-by-band signal wavelengthnumber calculation unit 21 so as to obtain a target gain from thisaverage inputted signal power and the target output signal power.

In more detail, first of all, the band-by-band signal wavelength numbercalculation unit 21 calculates the number of wavelengths (m′_k) includedin each of the bands 41, 42 and 43 according to the following equation(1) on the basis of an inputted monitored value of inputted light power(Pin_mon_k) for each of the bands 41, 42 and 43, and ASE power (Pase_k)for each of the bands 41, 42 and 43 and specified signal power(Pin_design) acquired in advance.m′ _(—) k=(Pin_mon_(—) k−Pase_(—) k)/Pin_design  (1)

Moreover, the number of wavelengths (m′_k) obtained is quantized by theaforesaid threshold and the approximated number of wavelengths (m_k) isdetermined. For example, if 0.7≦m′_k<1.41, m_k=1, if 1.41≦m′_k<2.82,m_k=2, if 2.82≦m′_k<5.15, m_k=4, if 5.15≦m′_k<11.3, m_k=8, and if11.3≦m′_k<22.6, m_k=16. The maximum value of the approximated number ofwavelengths m_k is limited to the maximum number of wavelengths includedin each band. Moreover, it is preferable that, as mentioned above, thethreshold for obtaining the approximated number of wavelengths m_k has adifferent value for each of the bands 41, 42 and 43.

Following this, the band-by-band signal wavelength number calculationunit 21 sets the sum total of the approximated numbers of wavelengthsm_k in the respective bands 41, 42 and 43, thus obtained, as the totalnumber of wavelengths m in the signal band 40 and determines the signalinput power (Pin) per wavelength according to the following equation (2)on the basis of the calculated number of wavelengths m. In the followingequation (2), Pase represents the sum total of the ASE powers Pase_k forthe respective bands 41, 42 and 43.Pin=(Pin_mon_(—) k−Pase)/m  (2)

Furthermore, on the basis of the signal input power Pin per wavelength,thus obtained, and the target output signal power (Pout_target), thetarget gain calculation unit 22 determines a target gain (Gt) accordingto the following equation (3) (from No route of step S3 to step S5).Gt=Pout_target/Pin  (3)

The automatic gain control unit 20 changes the target gain(predetermined value) in the steady state to the target gain Gt thuscalculated (step S6). Subsequently, until a predetermined period of timet=b (time sufficiently slower than the control response time to be takenfor the system level adjustment) elapses (until t=b in the step S7), thetarget gain Gt is calculated in the band-by-band signal wavelengthnumber calculation unit 21 and the target gain calculation unit 22, andthe automatic gain control unit 22 adjusts (corrects) the pump power tothe EDF 11 at a high speed in accordance with the target gain Gt thusupdated in succession (changes the AGC target gain to Gt), andsuppresses the transient variation occurring at a high speed, forexample, on the order of 1 μs while following it (through routeindicated by “t<b” in step S7 to steps S5 and S6, and see a solid linewaveform 52 in FIG. 9) . For example, since an optical variableattenuator (not shown) for the output adjustment in the OADM node 100usually starts to work after the response of the optical amplifier 1,the aforesaid predetermined time t=b is a time to be taken until theoutput of the optical amplifier 1 and the response of the opticalvariable attenuator in the OADM node 100 become stable.

Thereafter, when the aforesaid predetermined time elapses, the automaticgain control unit 20 once fixes the target gain Gt to a value at thattime (step S8, and see reference mark B in FIG. 9) and, after carryingout the automatic gain control (feedback control) on the basis of theinput/output monitored values obtained by the input PD 17-4 and theoutput PD 19 which monitor the entire signal band 40 of the opticalamplifier 1, brings the target gain Gt gradually closer to a design gainvalue (predetermined value) for a time slower than the response times ofthe other active devices to be used for the transmission system (forexample, by 0.1 dB) (step S9, Yes route of step S10, and see referencemark C in FIG. 9). When the target gain Gt reaches the aforesaid designgain value, the automatic gain control unit 20 shifts to the control tobe implemented in the steady state (through Yes route of step S10 tostep S11).

Since, as described above, the target gain Gt is updated in successionon the basis of the number of signal wavelengths calculated for each ofthe SHB band 41, the gain deviation band 42 and the SRS band 43 so as tocarryout the gain adjustment (correction) at a high speed, as indicatedby the solid line 52 in FIG. 9, even if the inputted light power to theoptical amplifier 1 varies suddenly due to a rapid variation of thenumber of signal wavelengths stemming from the occurrence of a troubleor the like, it is possible to suppress the variation (transientvariation) of the output signal power level (residual signal powerlevel) to a minimum (that is, to realize the ALC), so the accumulationof the transient output power variation is suppressible. In FIG. 9, adotted-line waveform 53 depicts a transient variation of residual signallight level in the case of a conventional AGC control, and it is foundtherefrom that a larger transient variation occurs in comparison withthe control according to this embodiment.

Therefore, for example, as shown in FIG. 10, in the case of atransmission system employing the conventional AGC, when the maximumnumber of transmission wavelengths is 40 channels and the disconnectionoccurs with respect to 39 channels thereof, the variation quantity ofthe residual signal power decreases whenever it passes through theoptical amplifier 1 (see reference numeral 53), whereas, when theoptical amplifier (AGC) according to the present invention is applied tothe post amplifier 1 placed on the output side of the OADM node 100, theresidual signal power variation can be reduced to approximately 1/10(see reference numeral 54). This signifies improvement of approximately4 dB as optical SN ratio at the signal reception end (for example, theoptical amplifier 101 in FIG. 25).

Moreover, since the signal band 40 is divided into three to monitor theinputted light power of each of the bands 41, 42 and 43, unlike theconventional technique, there is no need to prepare a wavelengthdemultiplexer (DEMUX) made to perform demultiplexing in units ofwavelengths for obtaining the number of signal wavelengths, so thedegradation of the noise characteristic (NF) is supprssible.

[A1] Modification of Gain Control Method

Secondly, a description will be given hereinbelow of another example ofthe above-described gain control method.

The target gain Gt can be calculated on the basis of a test result onthe gain characteristics of the optical amplifier 1, whose states arethe wavelength allocation of before and after the variation of thenumber of signal wavelengths or all the wavelengths are inputted, forexample. The target gain Gt calculated in advance is held in the database 23 (see FIG. 2) in the optical amplifier 1 and the target gain Gtis updated (selected) referring to the database 23 when the wavelengthallocation and the number of signal wavelength vary.

In this case, for example, the target gain calculation method is made asfollows.

(1) In a case in which, in a state where the number of signalwavelengths in the SHB band 41 is large (the number exceeding apredetermined value remains), the number of the signal wavelengths inthe other bands 42 and 43 (particularly, the gain deviation band 42)decreases considerably (below a predetermined value), the target gain Gtis decreased from the gain value in the steady state.

(2) In a case in which the number of signal wavelengths in the SHB band41 decreases considerably (below a predetermined value), the target gainGt is increased from the gain value in the steady state.

(3) In a case in which the number of signal wavelengths before avariation in the number of wavelengths in the SHB band 41 is small (lessthan a specified value k), the target gain Gt is not changed.

(4) In a case in which the number of signal wavelengths in the SHB band41 decreases considerably (less than the specified value k) and thenumber of signal wavelengths does not exceed a specified value m, thetarget gain Gt is increased from the gain value in the stead state.

(5) In a case in which, from the state the number of signal wavelengthsin the entire signal band 40 exceeds the specified value n, the numberof signal wavelengths in other than the SRS band 43 does not exceed aspecified value p and a decision is made such that the number of signalwavelengths left in the SRS band 43 is small, the target gain Gt isincreased from the gain value in the steady state.

On the basis of the above-mentioned updating operations, the gainadjustment quantity is determined in accordance with the gain wavelengthcharacteristic of the optical amplifier 1.

(6) In a case in which, in a state where the number of signalwavelengths is large in a band whose average gain is high, the number ofsignal wavelengths in the other band decreases considerably, the targetgain Gt is increased from the gain value in the steady state.

(7) In a case in which, in a state where the number of signalwavelengths is large in a band whose average gain is low, the number ofsignal wavelengths in the other band decreases considerably, the targetgain Gt is decreased from the gain value in the steady state.

FIG. 12 shows an example of a result of calculation (target gain settingtable)of the target gain Gt for realizing the above-mentionedoperations. The target gain setting table 231 shown in FIG. 12 ispreviously held in the aforesaid database 23. In the table 231 shown inFIG. 12, “before” represents “state before variation of the number ofsignal wavelengths” and “after” represents “state after variation of thenumber of signal wavelengths”, while the black circle mark depicts thatthe number of signal wavelengths in the band 41, 42 or 43 exceeds agiven number of wavelengths (specified value), and the white circle markdenotes that the number of signal wavelengths in the band 41, 42 or 43does not exceed a given number of wavelengths (specified value).

Therefore, as one example, in this table 231, at the line indicated byreference mark A, before and after the variation of number ofwavelengths, in a situation that signal wavelengths, the number of whichexceeds a given number of wavelengths (many in number), remain in theSHB band 41 while signal wavelengths, the number of which does notexceed the given number of wavelengths (small in number), remain in eachof the gain deviation band 42 and the SRS band 43, the target gain Gt isdecreased and, likewise, at the line indicated by reference mark Btherein, before and after the variation of number of wavelengths, in asituation that signal wavelengths, the number of which does not exceedsthe given number of wavelengths (small in number), remain in each of theSHB band 41 and the SRS band 43 while signal wavelengths, the number ofwhich exceeds the given number of wavelengths (many in number), remainin the gain deviation band 42, the target gain Gt is increased.

In this case, when the number of signal wavelengths (wavelengthallocation) varies, the target gain calculation unit 22 (see FIG. 2)sees (makes a retrieval on) the aforesaid table 231 on the number ofsignal wavelengths (wavelength allocation) of each of the bands 41, 42and 43 obtained by the band-by-band signal wavelength number calculationunit 21 to select the corresponding target gain Gt, thus updating thetarget gain Gt in succession.

With reference to a flow chart of FIG. 13, a description will be givenhereinbelow of a gain control method according to this example.

First, also in this modification, in the steady state (in a state wherethe variation of the number of signal wavelengths is smaller than theallowable quantity), the automatic gain control unit 20 carries out theautomatic gain control on the basis of the input/output monitored valuesof the EDF 11 obtained by the PD 17-4 and the PD 19 so that the gain ofthe EDF 11 reaches a predetermined gain value (steps S1, S2 and, throughYes route of step S3, step S4). At this time, the band-by-band signalwavelength number calculation unit 21 of the gain adjustment unit 30counts the number of signal wavelengths of each of the bands 41, 42 and43 as mentioned above for monitoring the variation in the number ofsignal wavelengths (steps S2, S3).

In this state, for example, assuming that more-than-half (39 channels orthe like of 40 channels) of all the number of signal wavelengths are notinputted because of the occurrence of a trouble or the like so that theinputted light power to the optical amplifier 1 (monitored by the PD17-3) decreases by 3 dB (see the arrow 51 in FIG. 9), the target gaincalculation unit 22 confirms the wavelength allocation and the variationin the number of wavelengths on the basis of the information on thenumber of wavelengths for each of the bands 41, 42 and 43 obtained bythe band-by-band signal wavelength number calculation unit 21 and refersto the target gain setting table 231 of the database 23 on the basis ofthese for acquiring (selecting) the corresponding target gain Gt [gainadjustment quantity (pump source drive current value)] (through No routeof step S3 to step S5′).

The automatic gain control unit 20 carries out the pump power control inaccordance with the setting of the target gain Gt by the target gaincalculation unit 22 (changes the AGC target gain to Gt), therebysuppressing the transient variation occurring, for example, at a highspeed on the order of 1 μs while following it (step S6, and see thesolid line waveform 52 in FIG. 9). Following this, the automatic gaincontrol unit 20 implements the automatic gain control (feedback control)on the basis of the input/output monitored values obtained by the inputPD 17-4, made to monitor the entire signal band 40 of the opticalamplifier 1, and the output PD 19 (see FIG. 2). However, the gain is setas the target gain Gt.

Moreover, after the elapse of a given time of period, the automatic gaincontrol unit 20 brings the target gain Gt closer to the design gainvalue (predetermined value) more slowly (for example, by 0.1 dB) withrespect to the response time of the other active devices (step 9, Yesroute of step S10, and see reference mark C in FIG. 9) . When the targetgain Gt reaches the aforesaid design gain value, the automatic gaincontrol shifts the control to the control in the steady state (throughYes route of step S10 to step S11).

In the case of the upgrade by a Raman amplification, a monitor forcoping with this transient response can also be used as a tilt monitorof the Raman amplifier.

As described above, according to this modification, with simple andquick control in which the target gain Gt is selected/set from thetarget gain setting table 231 of the database 23 on the basis of thenumber of signal wavelengths calculated for each of the SHB band 41, thegain deviation band 42 and the SRS band 43, as well as theabove-described embodiment, it is possible to suppress the variation(transient variation) of the output signal power level (residual signalpower level) to a minimum, thereby enabling the suppression of theaccumulation of the transient output power variation.

Therefore, also in this modification, even if disconnection occurs withrespect to 39 channels of the maximum number of transmissionwavelengths, the variation quantity of the residual signal power issuppressible to approximately 1/10, and an improvement of approximately4 dB is achievable as the optical SN ratio at the signal reception end.

[A2] Modification of Band-by-band Input Light Monitoring unit 10

Although in the above-described embodiment the PD 17-1 and the PD 17-2monitor the inputted signal power for each of the SHB band 41 and theSRS band 43 in the signal band 40 and the inputted signal power of thegain deviation band 42 is calculated by subtracting the monitor valuesobtained by these PDs 17-1 and 17-2 from the monitor value obtained bythe PD 17-4, it is also possible to monitor the inputted signal powerthrough the use of a PD for each of the three bands 41, 42 and 43.

That is, for example, as shown in FIG. 14, a PD 17-1, a PD 17-2 and a PD17-3 are prepared for the SHB band 41, the gain deviation band 42 andthe SRS band 43, respectively, and the signal band 40 is divided intothree: the aforesaid bands 41, 42 and 43, through the use of an opticalcoupler (3-dB coupler) 13 and edge filters 16-1, 16-2, with the dividedbands being inputted to the PDs 17-1, 17-2 and 17-3.

In addition, for example, as shown in FIG. 15, likewise, it is possibleto monitor the inputted signal power of each of the bands 41, 42 and 43through the use of 3-dB couplers 13, 14, band-pass filters 15-1, 15-215-3 for the bands 41, 42 and 43 and PDs 17-1, 17-2, 17-3.

Both the configurations can eliminate the need for the calculation ofthe inputted signal power of the gain deviation band 42 mentioned aboveand, hence, a higher-speed control is realizable.

[A3] Others

Although in each of the above-described embodiments the signalwavelength band 40 is divided into three bands: the SHB band 41, thegain deviation band 42 and the SRS band 43, so as to obtain the numberof signal wavelengths of each of the divided bands 41, 42 and 43, it isalso appropriate that the signal wavelength band 40 is divided into two:the SHB band 41 and the other band (including the gain deviation band42) or the SRS band 43 and the other band (including the gain deviationband 42), thereby obtaining the number of signal wavelengths of each ofthe divided bands. Also in this case, it is expectable to exhibit ahigher residual signal light power variation suppression effect incomparison with the conventional technique.

Furthermore, although in the above-described embodiment the presentinvention is applied to the post amplifier 1, it is natural that thepresent invention is applicable to the preamplifier 3 or 5 (see FIG. 1).

[B] Description of Second Embodiment

Secondly, a description will be given hereinbelow of a low-cost gainequalization device providing a high-speed response. As described above,the main factors of the variation of the wavelength flatness of signalpower occurring due to the dynamic variation of the state of signal arethe SHB occurring in an EDFA, the gain deviation of an EDFA and the SRSoccurring in an optical transmission line. Moreover, the degree of theeffect of each of the factors largely varies according to wavelengthand, for example, in the case of the C band (1532 nm to 1563 nm), theeffects of the SHB, the gain deviation and the SRS increases withshorter wavelength, while longer wavelength provides greater effects ofthe SRS and the gain deviation and an intermediate wavelength bandprovides a greater gain deviation effect in comparison with the othertwo factors.

Therefore, to suppress the variation of gain wavelength characteristicinvolved in the variation of signal state (number of wavelengths andwavelength allocation), a sufficient effect can be obtained by dividingthe signal band according to the contents of an occurrence factor andcarrying out the compensation in units of bands without performinghigh-cost compensation in units of wavelengths. Accordingly, asmentioned above with reference to FIG. 3, the signal band 40 is dividedinto three bands of the SHB band 41, the gain deviation band 42 and theSRS band 43 and means (gain equalization device) for performing theoutput equalization of signal in units of the bands 41, 42 and 43 iseasily realized at a low cost, thus preventing the occurrence oftransmission error due to the variation in the number of wavelengths andwavelength allocation.

(B1) Description of Gain Equalization Device having High-Speed ResponseCharacteristic

FIG. 26 is a block diagram showing a configuration of a gainequalization device according to a second embodiment of the presentinvention. For example, the gain equalization device (band unit gainequalizer) 60 shown in FIG. 26 is made up of two circulators 601, 602,three types of reflection devices 603, 608, 609, two WDM couplers 604,610 and three variable optical attenuators (VOA) 605, 606, 607. The VOA605 is connected to one output port of the circulator 601, and thereflection devices 608 and 609 are connected in series to the VOA 605,and one input port of the circulator 602 is connected to the output ofthe latter-stage reflection device 609. Moreover, the reflection device603 is connected to the other output port of the circulator 601, and theWDM coupler 604 is connected to the reflection device 603, and the VOA606 is connected to one output of the WDM coupler 604 and the VOA 607 isconnected to the other output thereof, and the outputs of these VOAs 606and 607 are connected to the WDM coupler 610, and the output of the WDMcoupler 610 is connected to the other input port of the latter-stagecirculator 602.

In this configuration, the circulator (first optical circulator) 601guides inputted WDM signal (main signal) to the reflection device 603and leads the inputted light from the reflection device 603 to the VOA605, and the reflection device (gain deviation band reflection device)603 reflects, of the inputted main signal from the circulator 601, lightwith wavelengths in the aforesaid gain deviation band (which willhereinafter be referred to equally as an “intermediate band”) 42 andpermits light with the other wavelengths (i.e., light in the aforesaidSHB band 41 and SRS band 43) to pass toward the WDM coupler 604 and, forexample, it can be made as a reflection type grating having thiswavelength reflection/transmission characteristic (this also applies tothe other reflection devices 608 and 609).

Each of the VOAs 605, 606 and 607 is for adjusting the degree ofattenuation of inputted light to adjust the output light power, and thedegree of attenuation of each of the VOAs 605, 606 and 607 isindividually adjusted by a control unit 70 (gain equalization controlunit 74) which will be described later with reference to FIG. 28 or 29.Each of the VOAs 605, 606 and 607 has a sufficiently high response speed(on the order of microsecond) (for example, ˜several tens μs) incomparison with the response speed of the power compensation of OADMnode 100 (approximately ˜10 ms) and, for this reason, in the followingdescription, they will sometimes referred to as high-speed VOAs 605,606, 607).

The WDM coupler (band separation device) 604 separates the inputtedlight passing through the aforesaid reflection device 603 into light inthe SHB band 41 and light in the SRS band 43 to output the light in theSHB band 41 to the VOA 607 and the light in the SRS band 43 to the VOA606, thus individually carrying out the output power adjustment on thelight in the SHB band 41 and the light in the SRS band 43 in the VOAs606 and 607. Incidentally, conversely, it is also acceptable to outputthe light in the SHB band 41 to the VOA 606 and the light in the SRSband 43 to the VOA 607.

The WDM coupler (band coupling device) 610 is for coupling (combining)the output lights of the VOA 606 and the VOA 607 to output the coupledlight to the circulator 602, and the circulator (second opticalcirculator) 602 is for leading the inputted light from the reflectiondevice 609 side to an output port and further for leading the inputtedlight from the WDM coupler 610 side to the VOA 605 (reflection devices608 and 609) side. That is, in this embodiment, the lights in the SHBband 41 and the SRS band 43 are guided through this circulator 602 tothe reflection device 609 side.

Moreover, the reflection device (SHB band reflection device) 608 is madeto reflect the light in the SHB band 41 and permit the light with theother wavelengths to pass, and the reflection device (SRS bandreflection device) 609 is made to reflect the light in the SRS band 43and permit the light with the other wavelengths to pass. Therefore, inthis embodiment, the light from the VOA 605 (that is, the light in thegain deviation band 42) passes through the reflection devices 608 and609 and is led to the circulator 602, while the light passing throughthe VOAs 606, 607, the WDM coupler 610 and the circulator 602 (that is,the light in the SHB band 41 and in the SRS band 43) is reflected by thereflection devices 608 and 609 to be again led to the circulator 602.

That is, the circulators 601, 602, the reflection devices 603, 608, 609and the WDM couplers 604, 610 function as a band division means todivide the signal wavelength band 40 of inputted wavelength multiplexedsignal into the three bands of the SHB band 41, the intermediate band 42and the SRS band 43, while the VOA 607 functions as a variable opticalattenuator for the SHB band 41, the VOA 605 function as a variableoptical attenuator for the intermediate band (gain deviation band) 42,and the VOA 606 functions as a variable optical attenuator for the SRSband 43, and these VOAs 605, 606 and 607 realizes an adjustment means toadjust the output light power in units of the divided bands 41, 42 and43.

With this configuration, in the gain equalization device 60 according tothis embodiment, inputted light propagates along optical paths 83, 81and 82 shown in FIG. 27 for each of the aforesaid bands 41, 42 and 43 sothat the adjustment of the output light power can be made independentlythrough the VOAs 605, 606 and 607 for each of the bands 41, 42 and 43.

That is, the inputted light to the gain equalization device 60 is firstled through the circulator 601 to the reflection device 603 and, asindicated by the optical path 81, the light in the gain deviation band42 is reflected by the reflection device 603 and again led to thecirculator 601 and further led through the VOA 605 and the reflectiondevices 608 and 609 to the circulator 602 and outputted from the outputport thereof.

On the other hand, as indicated by the optical path 83, the light in theSHB band 41, together with the light in the SRS band 43, passes throughthe reflection device 603 and is guided to the WDM coupler 604 and thenled to the VOA 607 and further led through the VOA 607, the WDM coupler610, the circulator 602 and the reflection device 609 to the reflectiondevice 608. In addition, it is reflected by the reflection device 608and passes through the reflection device 609 and outputted from theoutput port of the circulator 602.

Furthermore, as indicated by the optical path 82, the light in the SRSband 43, together with the light in the SHB band 41, passes through thereflection device 603 and is led to the WDM coupler 604 and then led tothe VOA 606 by the WDM coupler 604 and further guided through the VOA606, the WDM coupler 610 and the circulator 602 to the reflection device609 and reflected by this reflection device 609 to be outputted from theoutput port of the circulator 602.

Accordingly, by individually adjusting (controlling) the degrees ofattenuation of the high-speed VOAs 605, 606 and 607 provided on theaforesaid optical paths 81, 82 and 83, it is possible to independentlyadjust the output light power for each of the gain deviation band 42,the SRS band 43 and the SHB band 41, which enables the compensation(equalization) for the gain deviation (output light power deviation),occurring in the signal wavelength band 40, at a high speed (on theorder of microsecond, instead of the order of millisecond in theconventional technique) in units of bands 41, 42 and 43 but not in unitsof wavelengths.

In consequence, there is no need to provide a high-cost high-speed VOAfor each of wavelengths, and the function is achievable by thepreparation of the VOAs for only the three bands 41, 42 and 43irrespective of the number of wavelengths of the signal wavelength band40, which enables realizing the gain equalization device 60 having ahigh-speed response characteristic at a low cost.

In this connection, the gain equalization device 60 according to thisembodiment is designed such that the approximately same losses arise inthe above-mentioned optical paths 81, 82 and 83.

Thus, in the gain equalization device 60, it is possible to suppress thedeviation (differences) among the light power losses in the bands 41, 42and 43 to a minimum, and the same type is applicable as the high-speedVOAs 605, 606 and 607, or the high-speed VOAs 605, 606 and 607 havingthe approximately same attenuation widths are usable, which can reducethe production cost and facilitate the control on the degree ofattenuation.

(B2) Description of Example of Application of Gain Equalization Device60

The gain equalization device 60 is applicable to the OADM node 100 and,for example, it can be provided at the latter stage of an add/drop unit110 in the OADM node 100 as shown in FIG. 28, or it can be provided atthe former stage of an add/drop unit 110 in the OADM node 100 as shownin FIG. 29.

That is, the OADM node 100 shown in FIGS. 28 and 29 is made up of theoptical add/drop unit 110 for carrying out the add/drop on signal with aspecified (arbitrary) wavelength of inputted WDM signal, an optical WDMcoupler 50, a gain equalization device 60 provided at the former stageor latter stage of the optical add/drop unit 110, and a control unit 70for controlling the gain equalization by the gain equalization device60, and the control unit 70 includes a band-by-band monitoring unit 71,a loss calculation unit 72, a storage unit 73 and a gain equalizationcontrol unit 74.

In this case, the optical add/drop unit 110 can be made, for example,using a combination of an AWG (Arrayed Waveguide Grating) and a VOA, awavelength selective switch (WSS) and others. Moreover, this opticaladd/drop unit 110 has a function to adjust the output signal poweraccording to wavelength of WDM signal, and when the signal state is asteady state (when the number of wavelengths and the wavelengthallocation do not vary), the automatic level control (ALC) isimplemented so that the signal with each wavelength outputted from theoptical add/drop unit 110 becomes predetermined power, thus carrying outthe output equalization for each wavelength.

The optical WDM coupler 50 is for performing the power-branch on aportion of the WDM signal, inputted from the optical add/drop unit 110or the optical amplifier (EDFA) 1, as a monitor light to output it tothe band-by-band monitoring unit 71 and further for outputting the leftlight to the gain equalization device 60.

Moreover, in the control unit 70, the band-by-band monitoring unit 71 ismonitoring the reception light power (average signal power perwavelength) of each of the SHB band 41, the gain deviation band 42 andthe SRS band 43 on the basis of the monitor light inputted from the afore said optical coupler 50, and this monitoring function is realizablewith, for example, a configuration equivalent to that of theband-by-band input monitoring means 10 mentioned above with reference toFIGS. 2, 14 or 15.

The storage unit 73 is for previously storing a target value of theaverage output signal power per wavelength (hereinafter referred toequally as “target output signal power”) in accordance with theinsertion position of the gain equalization device 60, and the losscalculation unit (difference detecting unit) 72 is for calculating(detecting) a difference (loss quantity) between the monitor result bythe band-by-band monitoring unit 71 and the target output signal powerstored in the storage unit 73 for each of the bands 41, 42 and 43.

Moreover, the gain equalization control unit 74 controls the gainequalization device 60, in more detail, controls the aforesaid VOAs 605,606 and 607 individually, so that the loss quantity obtained by theaforesaid loss calculation unit 72 reaches a minimum (the average outputsignal power becomes the target output signal power) for adjusting theloss quantities of the aforesaid bands 41, 42 and 43 individually, thuscompensating for the differences in loss (gain deviation) occurring inthe signal wavelength band 40 in units of the bands 41, 42 and 43instead of units of wavelengths.

With this configuration, in the OADM node 100 according to thisembodiment, since the response speed of the wavelength-by-wavelengthgain equalization control by the optical add/drop unit 110 is lowimmediately after a rapid variation of the signal state due toreconstruction of a wavelength path or the like, although the losscharacteristic of add/drop unit 110 remains constant (the output signalpower variation (gain deviation) is left in the signal wavelength band40) at the transient period, the control unit 70 monitors the inputtedsignal power for each of the inputted signal bands 41, 42 and 43 andcontrols the gain equalization device 60 (VOAs 605, 606, 607) on thebasis of the monitor result so that the average output power of each ofthe bands 41, 42 and 43 in the output signal of the gain equalizationdevice 60 becomes a predetermined target output power, thereby enablingthe compensation for the aforesaid gain deviation at a high speed.

Incidentally, although in the above-described configuration, theband-by-band monitoring unit 71 monitors the inputted signal power ofthe gain equalization device 60 (that is, carries out the feed forwardcontrol), it is also appropriate to monitor the output signal power ofthe gain equalization device 60 (that is, to carry out the feedbackcontrol).

(B3) Description of Control Method in OADM Node 100

Referring to FIGS. 30, 31 and 32, a detailed description will be givenhereinbelow of a method of controlling the aforesaid gain equalizationdevice 60 (operation of the control unit 70).

First, as shown in FIG. 31, the aforesaid control unit 70 acquires theaverage value of the inputted signal power (average power) for each ofthe bands 41, 42 and 43 from the band-by-band monitoring unit 71 (stepS21). The average power for each of the bands 41, 42 and 43 is outputtedto the loss calculation unit 72 and, for example, the loss calculationunit 72 first makes a comparison between the average power of theintermediate band 42 and the target output power on the signalwavelength band 40 in the storage unit 73 (that is, in this case, thesame target output power is set with respect to each of the bands 41, 42and 43) (step S22) to make a decision as to whether or not both coincidewith each other (step S23).

If the decision result shows no coincidence, then the loss calculationunit 72 determines a controlled variable for the gain equalizationdevice 60 (high-speed VOA 605) needed for the average power of theintermediate band 42 to reach the target output power until bothcoincide with each other (the decision in step S23 indicates YES), andthe gain equalization control unit 74 controls the high-speed VOA 605 onthe basis of the determined controlled variable (through NO route ofstep S23 to step S24).

On the other hand, when the average power of the intermediate band 42coincides with the aforesaid target output power, the loss calculationunit 72 then makes a comparison between the average power of the SHBband 41 and the aforesaid target output power (through YES route of stepS23 to step S25) to make a decision as to whether or not both coincidewith each other (step S26). If the decision result indicates nocoincidence, then the loss calculation unit 72 determines a controlledvariable for the gain equalization device 60 (high-speed VOA 607) neededfor the average power of the SHB band 41 to reach the target outputpower until both coincide with each other (the decision in step S26indicates YES), and the gain equalization control unit 74 controls thehigh-speed VOA 607 on the basis of the determined controlled variable(through NO route of step S26 to step S27).

Moreover, if the average power of the SHB band 41 coincides with theaforesaid target output power, the loss calculation unit 72 then makes acomparison between the average power of the SRS band 43 and theaforesaid target output power (through YES route of step S26 to stepS28) to make a decision as to whether or not both coincide with eachother (step S29). If the decision result indicates no coincidence, theloss calculation unit 72 determines a controlled variable for the gainequalization device 60 (high-speed VOA 606) needed for the average powerof the SRS band 43 to reach the target output power until both coincidewith each other (the decision in step S29 indicates YES), and the gainequalization control unit 74 controls the high-speed VOA 606 on thebasis of the determined controlled variable (through NO route of stepS29 to step S30).

Still moreover, if the average power of the SHB band 41 coincides withthe aforesaid target output power, the adjustment of the output signalpower of all the bands 41, 42 and 43 to the target output power reachescompletion, and the equalization (compensation) of the gain deviationoccurring in the signal wavelength band 40 is made. Incidentally,although the comparison and decision on the target output power is madein the order of the intermediate band 42→the SHB band 41→the SRS band43, the present invention is not limited this order.

FIGS. 30(A) and 30(B) show images of the variation of the output powerin each of the bands 41, 42 and 43 by the above-described gainequalization control. That is, in a case in which, as a result of therapid variation of the signal state, as shown in FIG. 30(A), each of theaverage powers of the SHB band 41 and the SRS band 43 exceeds the targetoutput light power and the average power of the intermediate band 42 islower than the target output power, when the gain equalization controlis implemented by the control unit 70 as described above, the averagepowers of all the bands 41, 42 and 43 are adjusted to the target outputpower at a high speed as shown in FIG. 30(B).

Furthermore, with reference to FIGS. 32(A) to 32(F), a description willbe given hereinbelow of power variation states of the signal wavelengthband 40 in the steady state of the OADM node 100 and in a state where avariation of the signal output condition occurs.

The signal to be inputted to the OADM node 100 has a wavelengthcharacteristic as shown in FIG. 32 (A) because of manufacturingdifferences of the optical amplifier 1 or the like. Since the opticaladd/drop unit 110 can achieve the follow-up in the steady state, theoutput deviation of the optical amplifier 1 is compensated for as shownin FIG. 32(B), and the wavelength characteristic of the signal powerbecomes flat. Therefore, in this state, the output of the gainequalization device 60 satisfies the target output power (coincides withthe target output power) as shown in FIG. 32(C), and the gainequalization is unnecessary.

Thereafter, let it be assumed that the number of wavelengths in the WDMsystem varies due to the reconstruction of wavelength paths. The outputdeviation of the optical amplifier 1 varies from a shape shown in FIG.32(A) to a shape shown in FIG. 32(D) due to SHB, SRS, operating pointshift stemming from the average AGC of the optical amplifier 1. At thistime, the response speed of the wavelength-by-wavelength outputequalization due to the wavelength-by-wavelength ALC in the opticaladd/drop unit 110 is low, and the follow-up becomes difficultimmediately after variation in the number of wavelengths.

Therefore, also in the output of the optical add/drop unit 110, anoutput deviation remains as shown in FIG. 32(E). However, since theresidual output deviation is gain-equalized by the gain equalizationdevice 60 in units of the bands 41, 42 and 43 as mentioned above, theoutput deviation is reduced as shown in FIG. 32(F). In this connection,although the output deviation of each of the bands 41, 42 and 43 remainsbecause it is not compensated for by the gain equalization device 60according to this embodiment, the influence thereof is smaller incomparison with the output deviation among the bands 41, 42 and 43.

When the above-described gain equalization function (gain equalizationdevice 60 and the control unit 70) is applied to all of or a portion ofthe OADM nodes 100, in the OADM node 100 equipped with the gainequalization device 60, the output light power of the signal wavelengthband 40 is adjusted to be a target output lower at a high speed in unitsof the bands 41, 42 and 43 and, hence, the accumulation of the outputdeviations does not occur and, even if the transmission is made in amulti-stage fashion, the occurrence of transmission error at thereception end is preventable.

For example, as shown in FIG. 33, assuming that the number of repeateroptical amplifiers in the WDM transmission system is approximately 30,in the case of the conventional configuration, the OADM node 100 cannotfollow, and the residual output deviations are accumulated intact andthe large attenuation of the signal power is confirmed in the case ofmulti-node propagation (see reference numeral 71). On the other hand,according to this embodiment, since the output signal power is adjustedto a target value for each OADM node 100, the degradation of the signalpower does not occur irrespective of the multi-node transmission (seereference numeral 702). Thus, it is known that an improvement ofapproximately 8.5 dB is obtainable in comparison with the conventionalconfiguration.

(B4) Description of Adjustment Method for WDM Transmission System UsingGain Equalization Device 60

Since the output wavelength characteristic of the optical amplifier(EDFA) 1 produces a factor of transmission error at a variation inwavelength state, it is desirable that the output wavelengthcharacteristic thereof is flat (no deviation). However, in fact, adeviation appears due to dispersion at manufacturing of EDFA 1, thewavelength characteristic of optical coponents of the OADM node 100 andothers. Accordingly, at the start of the WDM transmission system, theadjustment is made through the use of the aforesaid gain equalizationdevice 60 so as to compensate for the manufacturing dispersion.

That is, for example, as shown in FIG. 34, a WDM transmission system isconstructed in a manner such that an OADM node 100 and a gainequalization device 60 are interposed between two optical amplifiers(EDFA) 1 (1 a, 1 b) and, in a state where signal with specified power isinputted to the former-stage EDFA 1 a, the output wavelengthcharacteristic of the latter-stage EDFA 1 b is monitored for each of thebands 41, 42 and 43 as mentioned above and the loss adjustment (feedbackcontrol) on the gain equalization device 60 is made as well as the gainequalization control by the aforesaid control unit 70 so that theaverage output power of each band 41, 42, 43 of the monitored outputwavelength characteristic becomes a target output power given inadvance, thereby achieving the compensation for the manufacturingdispersion and others.

Thus, even in a case in which, with respect to the output wavelengthcharacteristics of the EDFAs 1 a and 1 b, a deviation occurs in thesignal wavelength band 40 due to the manufacturing dispersion of theEDFAs 1 a and 1 b, the wavelength characteristic of the optical parts ofthe OADM node 100 and others as shown in FIG. 35(A), the compensationfor the deviation for each of the bands 41, 42 and 43 can be made forflatness through the gain equalization control on the gain equalizationdevice 60 as shown in FIG. 35(B). Incidentally, also in this case, thecompensation for the deviation of the aforesaid signal wavelength band40 can be made through the feed forward control.

(B5) Description of Optical Amplifier (EDFA) 1 Using Gain EqualizationDevice 60

The above-mentioned gain equalization device 60 is also applicable tothe optical amplifier (EDFA) 1. FIG. 36 shows a configuration of theEDFA 1 in this case. The EDFA 1 shown in FIG. 36 is made up of, as amain signal transmission system, for example, two stages of EDFs(amplification mediums) 11A, 11B; an optical coupler 111, an opticalisolator 112 and a WDM coupler 113 provided on the input side of theformer-stage EDF 11A; a gain equalizer (GEQ) 114, a variable opticalattenuator (VOA) 115, a gain equalization device 60, a WDM coupler 116,an optical isolator 117 and a WDM coupler 118 placed between the EDF 11Aand the EDF 11B; and an optical isolator 119 and optical couplers 120,121 provided on the output side of the latter-stage EDF 11B.

In addition, as a control system, there are provided photodiodes (PD)122, 123, 124 serving as light-receiving elements, pump laser diodes(LD) 125, 126 serving as pump light producing means and an automaticgain control (AGC) unit 127, and for realizing the gain equalizationcontrol equivalent to that of the aforesaid control unit 70 (see FIGS.28 or 29), there are provided a band-by-band (individual band)monitoring unit 71, a loss calculation unit 72, a storage unit 73 and again equalization control unit 74.

In the aforesaid main signal transmission system, the optical coupler111 makes a power-branch on a portion of the inputted main signal (WDMlight) as a monitor light and outputs it to the PD 122, and the opticalisolator 112 permits the main signal light passing through the opticalcoupler 111 to pass only in one direction of the latter-stage WDMcoupler 113 side for preventing the reflection return to the opticalcoupler 111. Moreover, the WDM coupler 113 is made to couple the pumplight for the EDF 11A from the pump LD 125 with the main signal in frontof the EDF 11A for inputting them to the EDF 11A.

The EDF 11A is for amplifying the main signal light by receiving thepump light produced by the aforesaid pump LD 125, and GEQ 114 has afixed gain equalization characteristic and is for carrying out the gainequalization in units of wavelengths with respect to the main signalfrom the EDF 11A, and the VOA 115 is for adjusting the main signal powerafter the gain equalization by the GEQ 114 in a manner such that theattenuation factor thereof is adjusted.

The gain equalization device 60 is for carrying out the gainequalization on gain deviation, which occurs in the signal wavelengthband 40 of the inputted main signal, in units of the SHB band 41, theintermediate band 42 and the SRS band 43 as described above. However, inthis embodiment, with respect to the response speed of the gainequalization operation, a device (high-speed VOA 605, 606, 607) having ahigher speed than the response speed of wavelength-by-wavelength ALC inthe OADM node 100 is usable, or a lower-speed device is also available.Incidentally, the gain equalization device 60 is inserted between theEDF 11A and the EDF 11B. This is because, considering the characteristic(noise characteristic, amplification efficiency) of the entire opticalamplifier 1, a better characteristic is obtainable in comparison with acase in which the gain equalization device 60 is provided at the formerstage of the EDF 11A or at the latter stage of EDF 11B.

The optical coupler 116 is for carrying out a power-branch on a portionof output light of the gain equalization device 60 as a monitor light tooutput it to the PD 123, and the optical isolator 117 permits the mainsignal passing through the optical coupler 116 to pass in only onedirection of the latter-stage WDM coupler 118 side for preventing thereflection return to the optical coupler 116. Moreover, the WDM coupler118 couples the excitation for the EDF 11B from the pump LD 126 with themain signal light in front of the EDF 11B to input them to the EDF 11B.

The EDF 11B is for amplifying the inputted main signal light byreceiving the pump light produced by the aforesaid pump LD 126, and theoptical isolator 119 permits the main signal light from the EDF 11B topass in only one direction of the latter-stage optical coupler 120 sidefor preventing the reflection return to the EDF 11B. Moreover, theoptical coupler 120 carries out a power-branch on a portion of the mainsignal light passing through the optical isolator 119 as a monitor lightto output it to the PD 124, and the optical coupler 121 carries out apower-branch on a portion of the main signal light passing through theoptical coupler 120 as a monitor light to output it to the band-by-bandmonitoring unit 71.

Furthermore, in the aforesaid control system, the PDs 122, 123 and 124are for outputting electric signals (voltage signals) corresponding tothe light reception quantities (power) of the monitor light inputtedfrom the optical couplers 111, 116 and 120 to the AGC unit 127, and thepump LDs 125 and 126 are for producing the pump light for the EDFs 11Aand 11B, and the AGC unit 127 is made to individually control the pumpLDs 125 and 126 on the basis of the voltage signal (i.e., power monitorvalue) corresponding to the light reception power (total power) from thePDs 122, 123 and 124, that is, on the basis of the input/output lightpower of the EDFs 11A and 11B and the target gain, thus controlling thegain of the EDFA 1 to a constant value.

In addition, the band-by-band monitoring unit 71, the loss calculationunit 72, the storage unit 73 and the gain equalization control unit 74are similar to those described above with reference to FIGS. 28 and 29,and the band-by-band monitoring unit 71 is made to divide monitor lighton the main signal light from the optical coupler 121 into three bandsof the SHB band 41, the intermediate band 42 and the SRS band 43 foracquiring the main signal power (average power) for each of the bands41, 42 and 43 and outputting it to the loss calculation unit 72.

The storage unit 73 is for previously storing a target output power perwavelength for the EDFA 1, i.e., a target output power (average power)on the signal wavelength band (SHB band 41, intermediate band 42 and SRSband 43) 40 of the main signal, and the loss calculation unit 72 makes acomparison between the average power of each of the bands 41, 42 and 43and the target output power in the storage unit 73 for obtaining a lossquantity of each of the bands 41, 42 and 43. Moreover, the gainequalization control unit 74 individually controls the VOAs 605, 606 and607 of the gain equalization device 60 in accordance with the lossquantity obtained by the loss calculation unit 72 so that the mainsignal output power becomes the target output power in units of thebands 41, 42 and 43, thereby compensating for the gain deviationoccurring in the signal wavelength band 40.

In the EDFA 1 thus configured, the AGC unit 127 controls the outputlight powers of the pump LDs 125 and 126 on the basis of the result,obtained by monitoring the total power of the input/output signal of therespective EDFs 11A and 11B through the use of the PDs 122, 12,3 and124, and the target gain for achieving the average AGC, and through theuse of the band-by-band monitoring unit 71, the loss calculation unit72, the storage unit 73 and the gain control unit 74, a loss quantity isobtained with respect to the target output power for each of the bands41, 42 and 43 and the loss quantity of the gain equalization device 60is adjusted for each of the bands 41, 42 and 43 in accordance with theobtained loss quantity so that the main signal power of each of thebands 41, 42 and 43 becomes the aforesaid target output power, therebysuppressing the gain deviation occurring in the signal wavelength band40.

(B6) Description of Method of Controlling the aforesaid EDFA 1 HavingGain Equalization Device 60

Furthermore, referred to FIGS. 37 and 38, a detailed description will begiven of gain equalization control in the EDFA 1 having theconfiguration described above with reference to FIG. 36. In the EDFA 1according to this embodiment, considering the stability of control, thegain equalization control by the gain equalization device 60 isimplemented at a sufficiently lower response speed (approximately 1second) in comparison with the wavelength-unit output equalizationcontrol in the OADM node 100.

First of all, in the EDFA 1, the average AGC is carried out in the AGCunit 127. That is, as shown in FIG. 37, the total power of theinput/output signal of the EDFs 11A and 11B is acquired (monitored) bythe PDs 122, 123 and 124 (step S31) and the average gain is obtained onthe basis of the monitor result in the AGC unit 127 so that a comparisonis made between this average gain and an AGC target gain stored inadvance in a memory (not shown) (step S32) to make a decision as towhether or not the average gain obtained by the aforesaid monitorreaches the target gain (step S33).

If the decision result shows that the average gain obtained by themonitor does not reach the target gain (if the decision in the step S33indicates NO), the AGC unit 127 individually controls the pump PDs 125and 126 until it reaches the target gain (until the decision in the stepS33 indicates YES) for properly adjusting the pump power (through NOroute of step S33 to step S34).

On the other hand, if the aforesaid average gain reaches the target gain(when the decision in the step S33 indicates YES), then the gainequalization control is implemented by the gain equalization device 60.That is, the band-by-band monitoring unit 71 acquires the average poweron each of the bands 41, 42 and 43 (through YES route of step S33 tostep S35), and the loss calculation unit 72 makes a comparison, forexample, between the average power of the intermediate band 42 and thetarget output power (step S36) to make a decision as to whether or notthis average power reaches the target output power (step S37).

If the decision shows that it does not reach the target output power (ifthe decision in the step S37 indicates NO), then the loss calculationunit 72 obtains the difference (i.e., loss quantity) from the targetoutput power until it reaches the target output power (until thedecision in the step S37 indicates YES), and the gain equalizationcontrol unit 74 adjusts the attenuation quantity of the VOA 605 of thegain equalization device 60 so that this loss quantity becomes at aminimum, thus adjusting (compensating for) the loss quantity on theintermediate band 42 (step S38).

On the other hand, If the decision shows that the average power on theintermediate band 42 reaches the target output power (if the decision inthe step S37 indicates YES), then the loss calculation unit 72 makes acomparison, for example, between the average power of the SHB band 41and the target output power (step S39), thereby making a decision as towhether or not this average power reaches the target output power (stepS40).

If the decision shows that the average power of the SHB band 41 does notreach the target output power (if the decision in the step S40 indicatesNO), then the loss calculation unit 72 obtains a difference (lossquantity) from the target output power until it reaches the targetoutput power (until the step S40 indicates YES), and the gainequalization control unit 74 adjusts the attenuation quantity of the VOA607 of the gain equalization device 60 so that this loss quantitybecomes at a minimum, thereby adjusting (compensates for) the lossquantity on the SHB band 41 (step S41).

On the other hand, if the average power on SHB band 41 reaches thetarget output power (when the decision in the step S40 indicates YES),then the loss calculation unit 72 makes a comparison between the averagepower of the SRS band 43 and the target output power (step S42) to makea decision as to whether or not this average power reaches the targetoutput power (step S43).

If the decision shows that the average power of the SRS band 43 does notreach the target output power (if the decision in the step S43 indicatesNO), then the loss calculation unit 72 obtains a difference (lossquantity) from the target output power until it reaches the targetoutput power (until the step S43 indicates YES), and the gainequalization control unit 74 adjusts the attenuation quantity of the VOA606 of the gain equalization device 60 so that this loss quantitybecomes at a minimum, thereby adjusting (compensates for) the lossquantity on the SRS band 43 (step S44).

Moreover, if the average power of the SRS band 43 reaches the targetoutput power (if the step S43 indicates YES), the average powers of allthe bands 41, 42 and 43 are adjusted to the target output power, and thecompensation for the gain deviation of the signal wavelength band 40 isachievable, thus forming flatness. Incidentally, in this embodiment,although the adjustment is made in the order of the intermediate band42→the SHB band 41→the SRS band 43, the present invention is not limitedthis order.

Furthermore, referring to FIG. 38, a description will be given hereinbelow of, in a case in which the above-described gain equalizationcontrol is applied to the EDFA 1, a steady state and a state of powervariation of the signal wavelength band 40 at the occurrence of avariation of the signal light state.

The signal power outputted from the EDFA 1 has a wavelengthcharacteristic shown in FIG. 38(A) due to the manufacturing dispersionor the like if the gain equalization is not implemented. In the steadystate, since the flow-up of the gain equalization operation is feasibleeven using a low-speed gain equalization device 60, the output lightpower deviation of the EDFA 1 is equalized in units of the bands 41, 42and 43 as shown in FIG. 38(B). At this time, the residual outputdeviation is output-equalized as shown in FIG. 38(C) by means of thewavelength-by-wavelength ALC function of the OADM node 100.

Following this, let it be assumed that the signal state (the number ofwavelengths and the wavelength allocation) of the WDM transmissionsystem varies due to the reconstruction of wavelength paths or the likeso that the output signal of the EDFA 1 varies from the wavelengthcharacteristic shown in FIG. 38(B) to the wavelength characteristicshown in FIG. 38(D). Thus, an operating point shift occurs from theoperating state shown in FIG. 38(D) due to the average AGC by the AGCunit 127 of the EDFA 1 so that the wavelength characteristic varies. Atthis time, since the gain equalization device 60 is adjusted so that thegain deviation decreases as shown in FIG. 38(B) at the time of thesteady state, the variation of the wavelength characteristic due to theoperating point shift becomes relatively small as shown in FIG. 38(E).

In this case, although the wavelength-by-wavelength output equalizationcontrol by the wavelength-by-wavelength ALC function of the OADM node100 cannot follow it, since the gain deviation is suppressed by the gainequalization device 60 as mentioned above, the variation of thewavelength characteristic due to the variation in the number ofwavelengths is small and the accumulation of the output deviation isalso small as shown in FIG. 38(F).

Therefore, it is possible to suppress the accumulation of the outputdeviation occurring in each node (EDFA 1) constituting the WDMtransmission system, and further to prevent the occurrence oftransmission error at the reception end even when the transmission ismade in a multistage system. Incidentally, the EDFA 1 having theabove-described gain equalization device 60 is applicable to all theoptical amplifiers constituting the WDM transmission system and, forexample as shown in FIG. 39, it can be applied to a portion of theoptical amplifiers in a range where the transmission error does notappear at the reception end. In FIG. 39, reference numeral 1A representsan optical amplifier (EDFA) equipped with the gain equalization device60, while reference numeral 1 depicts an existing optical amplifier(EDFA) which does not include the gain equalization device 60.

(B7) Description of Modification of Gain Equalization Device 60.

Furthermore, a description will be given hereinbelow of a modificationof the above-described gain equalization device 60. In particular, whenthe above-described gain equalization device 60 does not require ahigh-speed response characteristic, the application of theconfigurations according to the following first and second modificationsis advantageous in scale, cost and others.

(B7.1) First Modification

FIG. 40 is a block diagram showing a first modification of theabove-described gain equalization device 60. The gain equalizationdevice 60 shown in FIG. 40 is made up of an edge filter 611 serving as ahigh pass filter to permit a longer-wavelength side light to passinstead of a specified wavelength, an edge filter 612 serving as a lowpass filter to permit a shorter-wavelength side light to pass instead ofa specified wavelength, a variable optical attenuator (VOA) 613 and atransmission wavelength control unit 614.

In this configuration, for example, as shown in FIG. 41, the edge filter(first edge filter) 611 has a leading edge (slope) portion 620 of atransmission band (see a portion indicated by oblique lines) withrespect to wavelengths in the vicinity of the boundary between the SHBband 41 and the intermediate band 42, and shows a characteristic oftransmission of light with the longer-wavelength side wavelengths fromthis leading edge portion 620, with this leading edge portion 620 beingshifted (wavelength-shifted) on the wavelength axis by the transmissionwavelength control unit 614.

In addition, for example, as shown in FIG. 42, the edge filter (secondedge filter) 612 has a trailing edge (slope) portion 630 of atransmission band (see a portion indicated by oblique lines) onwavelengths in the vicinity of the boundary between the intermediateband 42 and the SRS band 43, and shows a characteristic of transmissionof light with the shorter-wavelength side wavelengths from this leadingedge portion 630, as well as the aforesaid edge filter 611, with thistrailing edge portion 630 being shifted (wavelength-shifted) on thewavelength axis by the transmission wavelength control unit 614.

Therefore, for example, as shown in FIG. 43, the combined transmissionband characteristic of these edge filters 611 and 612 becomes atrapezoid-like characteristic extending from the intermediate band 42 tothe SHB band 41 and the SRS band 43 and, in a manner such that therespective edge portions 620 and 630 are individually properlywavelength-shifted by the transmission wavelength control unit 614, thetransmission light quantities of the SHB band 41 and the SRS band 43 arechangeable.

The transmission wavelength control unit 614 is made to carry out theaforesaid wavelength shift by individually changing the angles of thinfilm filter of the edge filters 611 and 612, and the VOA 613 is made toadjust the output power of light passing through the edge filters 611and 612 with the attenuation quantity thereof being adjusted.

That is, in this embodiment, the aforesaid edge filters 611 and 612function as a band division means to divide the signal wavelength band40 of inputted wavelength multiplexed light into three bands of thebands 41, 42 and 43, and the aforesaid VOA 613 and transmissionwavelength control unit 614 function as an adjustment means to adjustthe output light power in units of the divided bands 41, 42 and 43.

With this configuration, the gain equalization device 60 controls theedge filters 611 and 612 to individually carry out the wavelength-shifton the edge portions 620 and 630 of the transmission bands, thusindependently controlling the output light powers of the SHB band 41 andthe SRS band 43.

Therefore, in comparison with the configuration described above withreference to FIG. 26, the gain equalization device 60 is realizable witha simpler configuration and at a lower cost.

(B7.2) Second Modification

FIG. 44 is a block diagram showing a second modification of theabove-described gain equalization device 60. The gain equalizationdevice 60 shown in FIG. 44 is made up of two fiber gratings 615 and 616arranged in series to each other, transmission characteristic controlmeans 615 a and 616 a provided in corresponding relation to these fibergratings 615 and 616, a transmission quantity control unit 617 and avariable optical attenuator (VOA) 618.

In this configuration, the input side grating (first transmission typefiber grating) 615 has a transmission (loss) characteristic 640 with agiven spread from the shortest-wavelength side of the signal wavelengthband 40, for example, as shown in FIG. 45, for providing a loss to thewavelengths of the SHB band 41, and the output side grating (secondtransmission type fiber grating) 616 has a transmission (loss)characteristic 650 with a given spread from the longest-wavelength sideof the signal wavelength band 40, for example, as shown in FIG. 46, forproviding a loss to the wavelengths of the SRS band 43. Therefore, thecombined transmission characteristic of these fiber gratings 615 and616, i.e., the transmission characteristic of the gain equalizationdevice 60, becomes as shown in FIG. 47. A transmission type such aslong-period type or slant type is employable as each of these fibergratings 615 and 616.

The transmission characteristic control means 615 a is for applying atemperature or a pressure to the corresponding fiber grating 615 tochange the transmission characteristic of the grating for controllingthe loss quantity of the SHB band 41, and a peltier element can be usedtherefor in the case of providing the temperature variation. Moreover,likewise, the transmission characteristic control means 616 a is forapplying a temperature or a pressure to the corresponding fiber grating616 to change the transmission characteristic of the grating forcontrolling the loss quantity of the SRS band 43.

The transmission quantity control unit 617 is for independentlycontrolling the temperature or pressure to be applied to the fibergratings 615 and 616 by the transmission characteristic control means615 a and 616 a so as to independently control the loss quantities ofthe fiber gratings 615 and 616, thereby independently controlling thetransmission light quantity.

That is, in this embodiment, the a fore said fiber gratings 615 and 616function as a band division means to divide the signal wavelength band40 of the inputted wavelength multiplexed light into the bands 41, 42and 43, while the VOA 618, the transmission characteristic control means615 a, 616 a and the transmission quantity control unit 617 function asan adjustment means to adjust the output light power in units of thedivided bands 41, 42 and 43.

In the gain equalization device 60 thus configured, the transmissionquantity control unit 617 can independently control the transmissioncharacteristic control means 615 a and 616 a so as to individuallycontrol the loss quantities of the SHB band 41 and the SRS band 43. Alsoin this case, in comparison with the configuration described above withreference to FIG. 26, the gain equalization device 60 is realizable witha simpler configuration and at a lower cost.

Incidentally, although the gain equalization device 60 according to thefirst modification and the second modification can be replaced with anexisting DGEQ, the DGEQ requires a higher cost compared with this device60 and produces a larger insertion loss and, hence, advantage of theemployment of the DGEQ is small.

(B7.3) Third Modification

FIG. 48 is a block diagram showing a third modification of theabove-described gain equalization device 60. The gain equalizationdevice 60 shown in FIG. 48 is made up of WDM couplers 621, 622, 623,624, 630, 631, 632, 633, and variable optical attenuators (VOAs) 625,626, 627, 628, 629. If the gain equalization device 60 requires ahigh-speed response characteristic, as the VOAs 625, 626, 627, 628 and629, a high-speed VOA having a sufficiently higher response speed(˜several tens μs) in comparison with the response speed (˜10 ms) of thelevel compensation control in the OADM node 100 may be used as describedabove with reference to FIG. 26.

In this configuration, the WDM coupler (band separation or split device)621 functions as a band splitter to separate, of the inputted mainsignal light, the light corresponding to four wavelengths from theshortest-wavelength side of the intermediate band 42 to output it to theVOA 629 and to the light with the left wavelengths to the latter-stageWDM coupler 622. In this case, it has a band separation (loss)characteristic which does not produce a loss with respect to the lightwith wavelengths at both sides on the wavelength axis at the separationof the four wavelengths (in FIG. 48, “4skip0” signifies this).

In addition, the WDM coupler (band separation device) 622 also functionsas a band splitter and has a band separation (loss) characteristic(“4skip0”) equivalent to that of the a fore said band splitter 621, andis made to extract (separate), of the main signal from the band splitter621, the light corresponding to four wavelengths from thelongest-wavelength side of the intermediate band 42 to output it to theVOA 625 and to the light with the left wavelengths to the latter-stageWDM coupler 623.

Still additionally, the WDM coupler (band separation device) 623 alsofunctions as a band splitter and separate, of the main signal light fromthe aforesaid band splitter 622, the light left in the intermediate band42 to output it to the VOA 626 and to the light with the leftwavelengths (SHB band 41, the SRS band 43) to the latter-stage WDMcoupler 624. In this case, it has a band separation (loss)characteristic which produces, as a sacrifice, a loss with respect tothe light corresponding to four wavelengths at each of both sides on thewavelength axis at the separation of 16 wavelengths (in FIG. 48,“16skip4” signifies this).

That is, the gain equalization device 60 according to this embodiment isdesigned as the band splitter 623 to be used for separating the light ofthe intermediate band 42 such that, for employing a low-cost splitter ofthe aforesaid “16skip4”, the four wavelengths at both sides of theintermediate band 42 are separated in advance by the band splitters 621and 622.

Yet additionally, the WDM coupler (band separation device) 624 alsofunctions as a band splitter to split the inputted light from the bandsplitter 623 into the light in the SHB band 41 and the light in the SRSband 43 for outputting them to the VOAs 627 and 628. The WDM coupler(band coupling device) 630 combines the output lights (that is, thelights in the SHB band 41 and the SRS band 43) from the VOAs 627 and 628to output it to the WDM coupler 631. The WDM coupler (band couplingdevice) 631 is for combining the output light (i.e., light with 16 wavesin the intermediate band 42) of the VOA 626 with the output light (i.e.,lights in the SHB band 41 and the SRS band 43) of the WDM coupler 630.

The WDM coupler (band coupling device) 632 is for coupling the outputlight of the aforesaid WDM coupler 631 with the output light (lightcorresponding to four wavelengths from the shortest-wavelength side inthe intermediate band 42) of the VOA 629, and the WDM coupler (bandcoupling device) 633 is for coupling the output light of this WDMcoupler 632 with the output light (light corresponding to fourwavelengths from the longest-wavelength side in the intermediate band42) of the VOA 625.

Referring to FIG. 49, a description will be given hereinbelow of anoperation of the gain equalization device 60 thus configured. In FIG.49, (1) to (6) designate wavelength arrangements at places indicated by(1) to (6) in FIG. 48, respectively.

First of all, the main signal (see (1) in FIG. 49) inputted to the gainequalization device 60 arrives at the band splitter 621 where, of thelight, the light corresponding to four wavelengths on theshortest-wavelength side of the intermediate band 42 is separated andinputted to the VOA 629 without providing a loss to the otherwavelengths, while the light with the remaining wavelengths is inputtedto the latter-stage band splitter 622 (see (2) in FIG. 49).

In the band splitter 622, of the inputted main signal, the lightcorresponding to four wavelengths on the longest-wavelength side of theintermediate band 42 is separated and inputted to the VOA 625 withoutproviding a loss to the other wavelengths, while the light with theremaining wavelengths is inputted to the latter-stage band splitter 623(see (3) in FIG. 49).

In addition, the band splitter 623 splits the inputted main signal fromthe band splitter 622 into the light with wavelengths other than thefour wavelengths on each of the shortest-wavelength side andlongest-wavelength side of the intermediate band 42 and the light in theSHB band 41 and the SRS band 43, and outputs the former to the VOA 626(see (4) in FIG. 49) and outputs the latter to the band splitter 624.The band splitter 624 splits the inputted main signal light from theband splitter 623 into the light in the SHB band 41 and the light in theSRS band 41 to input them to the VOAs 627 and 628.

That is, the light of the intermediate band 42 is inputted to the VOAs625, 626 and 629, and the light of the SHB band 41 (or the SRS band 43)is inputted to VOA 627, and the light of the SRS band 43 is inputted tothe VOA 628. Moreover, by individually controlling the attenuationquantities of the VOAs 625, 626, 627, 628 and 629, it is possible toadjust the output light power deviation of the main signal light inunits of the bands 41, 42 and 43.

In this connection, the output lights (the lights of the SHB band 41 andthe SRS band 43) of the VOAs 627 and 628 are combined with each otherand inputted to the WDM coupler 631, where the combined light iscombined with the output light (light excluding the light correspondingto the four wavelengths on each of both the sides of the intermediateband 42) of the VOA 626 and outputted to the latter-stage WDM coupler632 (see (5) in FIG. 49). Moreover, the output light of the WDM coupler631 is combined with the output light (the light corresponding to thefour wavelengths on the shortest wavelength side of the intermediateband 42) of the VOA 629 in the WDM coupler 632, and the output light ofthis WDM coupler 632 is combined with the output light (the lightcorresponding to the four wavelengths on the longest-wavelength side ofthe intermediate band 42) of the VOA 625 in the WDM coupler 633 and thenoutputted (see (6) in FIG. 49).

As described above, with respect to the light with every wavelength inall the bands 41, 42 and 43 included in the signal wavelength band 40,without producing any loss [without making a sacrifice due to theaforesaid band separation characteristic) (which will be referred to as“no guard band”)], the adjustment of the output light powers becomesfeasible through the use of the VOAs 625 to 629. That is, by controllingthe attenuation quantities of the VOAs 625, 626 and 629, the outputlight power (loss) of the intermediate band 42 is adjustable, and bycontrolling the attenuation quantity of the VOA 627, the output lightpower (loss) of the SHB band 41 (or the SRS band 43) is adjustable, andby controlling the attenuation quantity of the VOA 628, the output lightpower (loss) of the SRS band 43 is adjustable.

Therefore, in comparison with the configuration described above withreference to FIG. 26, without employing the circulators 601, 602 and thereflection devices 603, 609, the gain equalization device 60 in units ofthe bands 41, 42 and 43 is realizable. Incidentally, the insertion lossof the gain equalization device 60 with this configuration isapproximately ˜5 dB.

(B7.4) Fourth Modification

FIG. 50 is a block diagram showing a fourth modification of theabove-described gain equalization device 60. The gain equalizationdevice 60 shown in FIG. 50 is made up of WDM couplers 641, 642, 646, 647and variable optical attenuators (VOAs) 643, 644, 645. Also in thismodification, in a case in which a high-speed response characteristic isrequired, as the VOAs 643, 644 and 645, a high-speed VOA having asufficiently higher response speed (˜several tens μs) in comparison withthe response speed (˜10 ms) of the level compensation control in theOADM node 100 may be used as described above with reference to FIG. 26.

In this configuration, the WDM coupler (band separation device) 641functions as a band splitter to separate the light of the SHB band 41from the inputted main signal for outputting it to the VOA 645 and tooutput the light with the remaining wavelengths to the latter-stage WDMcoupler 642. In this case, the band splitter 641 according to thismodification has a band separation (loss) characteristic which produces,as a sacrifice, a loss with respect to the light corresponding to twowavelengths at each of both sides on the wavelength axis at theseparation of 8 wavelengths (in FIG. 50, “8skip2” signifies this).

In addition, the WDM coupler (band separation device) 642 also functionsas a band splitter and has a band separation (loss) characteristic(“8skip2”) equivalent to that of the aforesaid band splitter 641, and ismade to separate, of the inputted light from the band splitter 641, thelight of the SRS band 43 to output it to the VOA 643 and to theremaining light of the intermediate band 42 to the VOA 644.

The WDM coupler (band coupling device) 646 is for coupling the outputlight (light of the intermediate band 42) of the VOA 644 with the outputlight (light of the SHB band 41) of the VOA 645, and the WDM coupler(band coupling device) 647 is for coupling the output light (lights ofthe SHB band 41 and the intermediate band 42) of this WDM coupler 646with the output light (light of the SRS band 43) of the VOA 643. Boththe WDM couplers 646 and 647 have a loss characteristic (“8skip2”)equivalent to those of the band splitters 641 and 642.

Referring to FIG. 51, a description will be given hereinbelow of anoperation of the gain equalization device 60 thus configured. In FIG.51, (1) to (5) depict the wavelength arrangements at places indicated by(1) to (5) in FIG. 50, respectively.

First of all, the main signal (see (1) in FIG. 51) inputted to the gainequalization device 60 arrives at the band splitter 641 where the lightof the SHB band 41 is separated and inputted to the VOA 645, and thelight with the remaining wavelengths is inputted to the latter-stageband splitter 642 (see (2) in FIG. 51). However, at this time, twochannels from the shortest-wavelength side in the intermediate band 42suffer a loss as a sacrifice due to the band separation characteristic(“8skip2”) of the band splitter 641 (a guard band exists).

In the band splitter 642, from the inputted light from the aforesaidband splitter 641, the light of the SRS band 43 is separated andoutputted to the VOA 643, and the light with the remaining wavelengthsis outputted to the VOA 644 (see (3) in FIG. 51). However, also in thiscase, two wavelength from the longest-wavelength side in theintermediate band 42 suffers a loss as a sacrifice due to the bandseparation characteristic (“8skip2”) of the band splitter 642.

From the above, the light of the SHB band 41 is inputted to the VOA 645,and the light of the intermediate band 42 is inputted to the VOA 644,and the light of the SRS band 43 is inputted to the VOA 643. Therefore,by individually controlling the attenuation quantities of the VOAs 645,644 and 643, a loss (output light power) for each of the bands 41, 42and 43 is adjustable.

The output lights (lights of the SHB band 41 and the intermediate band42) of the VOAs 644 and 645 are coupled in the WDM coupler 646 and thencoupled with the output light (light of the SRS band 43) of the VOA 643in the WDM coupler 647 and outputted.

With this configuration, in comparison with the above-describedconfiguration according to the third modification, although there arewavelengths (two wavelengths at each of both sides of the intermediateband 42, four wavelengths in total) (guard band) which becomes asacrifice to the aforesaid separation (loss) characteristic at the bandseparation, since there is no need to employ a high-performance bandsplitter (WDM coupler) with respect to the wavelength separationcharacteristic, the gain equalization device 60 is realizable at a lowercost.

(B8) Others

Although in each of the above-described embodiments the signalwavelength band 40 is divided into the SHB band 41, the gain deviationband 42 and the SRS band 43 and the gain compensation is made in unitsof the bands 41, 42 and 43, it is also appropriate that, for example,the signal wavelength band 40 is divided into two bands of the SHB band41 and the other band (including the gain deviation band 42), or the SRSband and the other band (including the gain deviation band 42), and thegain compensation is made in units of the divided bands.

1. A control apparatus for an optical amplifier comprising: an automaticgain control unit for controlling a gain of said optical amplifier to aconstant value based on power of incoming light and outgoing light ofsaid optical amplifier and a target gain; an individual band incominglight monitoring unit for dividing a signal wavelength band of theincoming light into at least a first band and second band: said firstband has tendency of decreasing power of the outgoing light at andecrease in the number of signal wavelengths and said second band, is asignal wavelength band other than said first band and, including a gaindeviation band in which power of the outgoing light varies mainly due toa wavelength deviation of the gain control by said automatic gaincontrol unit, and for monitoring power of the incoming light to saidoptical amplifier in each of the first and second bands; an individualband signal wavelength number calculation unit for obtaining the numberof signal wavelengths in each of the first and second bands based on aresult of the monitoring by the individual band incoming lightmonitoring unit; and a target gain correction unit for correcting saidtarget gain to be used in said automatic gain control unit based on aresult of the calculation by said individual band signal wavelengthnumber calculation unit at a variation of the number of signalwavelengths.
 2. The control apparatus according to claim 1, wherein saidindividual band incoming light monitoring unit divides said signalwavelength band into three bands: an SHB band, said gain deviation bandand SRS band, said SHB band being under dominance of a spectral holeburning (SHB) effect as said first band, and said SRS band being underdominance of a stimulated Raman scattering (SRS) effect occurring in anoutput transmission line of said optical amplifier as another band ofsaid first band, and monitors power of the incoming light in each ofsaid three bands.
 3. The control apparatus according to claim 2, whereinsaid signal wavelength band is a C band, and said SHB band is from 1530nm to 1540 nm, said SRS band is from 1555 nm to 1565 nm, and said gaindeviation band is a band interposed between said SHB band and said SRSband.
 4. The control apparatus according to claim 1, wherein saidindividual band signal wavelength number calculation unit divides valuesof power monitored for the individual divided bands by said individualband incoming light monitoring unit by a design value of power of signallight per signal wavelength, and determines a value closest to the nthpower of 2 from the resultant value obtained by the division as thenumber of signal wavelengths in each of the divided bands.
 5. Thecontrol apparatus according to claim 4, wherein a threshold to be usedfor the determination of the number of signal wavelengths is set on thebasis of a characteristic of output light power variation in each of thedivided bands.
 6. The control apparatus according to claim 5, whereinsaid threshold for said first band is set to be smaller than a thresholdfor said second band including said gain deviation band.
 7. The controlapparatus according to claim 1, wherein said target gain correction unitincreases said target gain, when recognizing, based on a result of thecalculation by said individual band signal wavelength number calculationunit, that the number of signal wavelengths in said first band becomessmaller than a predetermined value.
 8. The control apparatus accordingto claim 1, wherein said target gain correction unit decreases saidtarget gain, when recognizing, based on a result of the calculation bysaid individual band signal wavelength number calculation unit, that thenumber of signal wavelengths in said gain deviation band becomes smallerthan a predetermined value, in a state where the remaining number ofsignal wavelengths in said first band exceeds a predetermined value. 9.The control apparatus according to claim 1, wherein said target gaincorrection unit updates said target gain based on a result of thecalculation by said individual band signal wavelength number calculationunit until a predetermined period of time elapses from the occurrence ofa variation of the number of signal wavelengths and, after the elapse ofsaid predeterminded period of time, brings said target gain graduallycloser to a specified gain value.
 10. A control method for an opticalamplifier having an automatic gain control function to control a gain ofsaid optical amplifier to a constant value based on power of incominglight and outgoing light of said optical amplifier and a target gain,comprising the steps of: dividing a signal wavelength band of theincoming light into at least a first band and second band: said firstband has tendency of decreasing power of the outgoing light at adecrease in the number of signal wavelengths and said second band, is asignal wavelength band other than said band, including a gain deviationband in which power of the outgoing light varies mainly due to awavelength deviation of the gain control, and monitoring power of theincoming light in each of the divided bands; obtaining the number ofsignal wavelengths in each of the divided bands based on a result of themonitoring; and correcting said target gain to be used for the automaticgain control based on the number of signal wavelengths in each of thedivided bands at a variation of the number of signal wavelengths. 11.The control method according to claim 10, comprising: dividing saidincoming light signal wavelength band into three bands: an SHB band,said gain deviation band and an SRS band, said SHB band being underdominance of a spectral hole burning (SHB) effect as said first band,and said SRS band being under dominance of a stimulated Raman scattering(SRS) effect occurring in an output transmission line of said opticalamplifier as another band of said first band, and monitoring power ofincoming light in each of said three bands.
 12. The control methodaccording to claim 11, wherein said signal wavelength band is a C band,and said SHB band is from 1530 nm to 1540 nm, said SRS band is from 1555nm to 1565 nm, and said gain deviation band is a band interposed betweensaid SHB band and said SRS band.
 13. The control method according toclaim 10, comprising: dividing values of power monitored for theindividual divided bands by a design value of power of signal light persignal wavelength, and determining a value closest to the nth power of 2from the resultant value obtained by the division as the number ofsignal wavelengths in each of the divided bands.
 14. The control methodaccording to claim 13, wherein a threshold to be used for thedetermination of the number of signal wavelengths is set on the basis ofa characteristic of output light power variation in each of the dividedbands.
 15. The control method according to claim 14, wherein saidthreshold for said first band is set to be smaller than a threshold forsaid second band including said gain deviation band.
 16. The controlmethod according to claim 10, wherein said target gain is increased whenthe number of signal wavelengths in said first band becomes smaller thana predetermined value.
 17. The control method according to claim 10,wherein said target gain is decreased when the number of signalwavelengths in said gain deviation band becomes smaller than apredetermined value, in a state where the remaining number of signalwavelengths in said first band exceeds a predetermined value.
 18. Thecontrol method according to claim 10, wherein said target gain isupdated based on the number of signal wavelengths for each of thedivided bands until a predetermined period of time elapses from theoccurrence of a variation of the number of signal wavelengths and, afterthe elapse of said predetermined period of time, said target gain isbrought gradually closer to a specified gain value.
 19. An opticaltransmission apparatus comprising: an automatic gain control unit forcontrolling a gain of an optical amplifier to a constant value based onpower of incoming light and outgoing light of said optical amplifier anda target gain; an individual band incoming light monitoring unit fordividing a signal wavelength band of the incoming light into at least afirst band and second band: said first band has tendency of decreasingpower of the outgoing light per channel at a decrease in the number ofsignal wavelengths or increasing power of the outgoing light per channelat an increase in the number of signal wavelengths and asid second band,is a signal wavelength band other than said band, including a gaindeviation band in which power of the outgoing light per channel variesmainly due to a wavelength deviation of the gain control by saidautomatic gain control unit, and for monitoring power of the incominglight in each of the divided bands; an individual band signal wavelengthnumber calculation unit for obtaining the number of signal wavelengthsin each of the divided bands based on a result of the monitoring by saidindividual band incoming light monitoring unit; and a target gaincorrection unit for correcting said target gain to be used by saidautomatic gain control unit based on a result of the calculation by saidindividual band signal wavelength number calculation unit at a variationof the number of signal wavelengths.
 20. The optical transmissionapparatus according to claim 19, wherein said individual band inputlight monitoring unit divides said signal wavelength band into threebands: an SHB band, said gain deviation band and an SRS band, said SHBband being under dominance of a spectral hole burning (SHB) effect assaid first band and said SRS band being under dominance of a stimulatedRaman scattering (SRS) effect occurring in an output transmission lineof said optical amplifier as another band of said first band , andmonitors power of the incoming light in each of said three bands. 21.The optical transmission apparatus according to claim 20, wherein saidSHB band is from 1530 nm to 1540 nm, said SRS band is from 1555 nm to1565 nm, and said gain deviation band is a band interposed between saidSHB band and said SRS band.
 22. The optical transmission apparatusaccording to claim 19, wherein said individual band signal wavelengthnumber calculation unit divides values of power monitored for theindividual divided bands by said individual band incoming lightmonitoring unit by a design value of power of signal light per signalwavelength, and determines a value closest to the nth power of 2 fromthe resultant value obtained by the division as the number of signalwavelengths in each of the divided bands.
 23. The optical transmissionapparatus according to claim 22, wherein a threshold to be used for thedetermination of the number of signal wavelengths is set on the basis ofa characteristic of output light power variation per channel in each ofthe divided bands.
 24. The optical transmission apparatus according toclaim 23, wherein said threshold for said first band is set to besmaller than a threshold for said second band including said gaindeviation band.
 25. The optical transmission apparatus according toclaim 19, wherein said target gain correction unit increases said targetgain, when recognizing, based on a result of the calculation by saidindividual band signal wavelength number calculation unit, that thenumber of signal wavelengths in said first band becomes smaller than apredetermined value.
 26. The optical transmission apparatus according toclaim 19, wherein said target gain correction unit decreases said targetgain, when recognizing, based on a result of the calculation by saidindividual band signal wavelength number calculation unit, that thenumber of signal wavelengths in said gain deviation band becomes smallerthan a predetermined value, in a state where the remaining number ofsignal wavelengths in said first band exceeds a predetermined value. 27.The optical transmission apparatus according to claim 19, wherein saidtarget gain correction unit updates said target gain based on a resultof the calculation by said individual band signal wavelength numbercalculation unit until a predetermined period of time elapses from theoccurrence of a variation of the number of signal wavelengths and, afterthe elapse of said predetermined period of time, brings said target gaingradually closer to a specified gain value.
 28. An optical amplifierhaving an automatic gain control function to control a gain to aconstant value based on power of incoming light and outgoing light and atarget gain, comprising: dividing a signal wavelength band of theincoming light into at least a first band and second band: said firstband has tendency of decreasing power of the outgoing light per channelat a decrease in the number of signal wavelengths or increasing power ofthe outgoing light per channel at an increase in the number of signalwavelengths and said second band, is a signal wavelength band other thansaid band, including a gain deviation band in which power of theoutgoing light varies mainly due to a wavelength deviation of the gaincontrol; monitoring power of the incoming light in each of the dividedbands; obtaining the number of signal wavelengths in each of the dividedbands based on a result of the monitoring; and correcting said targetgain to be used for the automatic gain control based on the number ofsignal wavelengths in each of the divided bands at a variation of thenumber of signal wavelengths.
 29. The optical amplifier according toclaim 28, comprising: dividing said signal wavelength band into threebands: an SHB band, said gain deviation band and an SRS band, said SHBband being under dominance of a spectral hole burning (SHB) effect assaid first band, and said SRS band being under dominance of a stimulatedRaman scattering (SRS) effect occurring in an output transmission lineof said optical amplifier as another band of said first band; andmonitoring power of incoming light in each of said three bands.
 30. Theoptical amplifier according to claim 29, wherein said SHB band is from1530 nm to 1540 nm, said SRS band is from 1555 nm to 1565 nm, and saidgain deviation band is a band interposed between said SHB band and saidSRS band.
 31. The optical amplifier according to claim 28, comprising:dividing values of power monitored for the individual divided bands by adesign value of power of signal light per signal wavelength; anddetermining a value closest to the nth power of 2 from the resultantvalue obatained by the division as the number of signal wavelengths ineach of the divided bands.
 32. The optical amplifier according to claim31, wherein a threshold to be used for the determination of the numberof signal wavelengths is set on the basis of a characteristic of outputlight power variation per channel in each of the divided bands.
 33. Theoptical amplifier according to claim 32, wherein said threshold for saidfirst band is set to be smaller than a threshold for said second bandincluding said gain deviation band.
 34. The optical amplifier accordingto claim 28, wherein said target gain is increased when the number ofsignal wavelengths in said first band becomes smaller than apredetermined value.
 35. The optical amplifier according to claim 28,wherein said target gain is decreased when the number of signalwavelengths in said gain deviation band becomes smaller than apredetermined value, in a state where the remaining number of signalwavelengths in said first band exceeds a predetermined value.
 36. Theoptical amplifier according to claim 28, wherein said target gain isupdated on the basis of the number of signal wavelengths for each of thedivided bands until a predetermined period of time elapses from theoccurrence of a variation of the number of signal wavelengths and, afterthe elapse of said predetermined period of time, said target gain isbrought gradually closer to a specified gain value.
 37. An individualband gain equalizer comprising: a band division means for dividing asignal wavelength band of an incoming wavelength multiplexed light intoat least a first band and second band: said first band has tendency ofdecreasing power of outgoing light of an optical amplifier per channelat a decrease in the number of signal wavelengths or increasing power ofthe outgoing light of said optical amplifier per channel at an increasein the number of signal wavelengths and said second band, is a signalwavelength band other than said first band, including a gain deviationband in which power of the outgoing light per channel varies mainly dueto a wavelength deviation of automatic gain control in the opticalamplifier; and an adjustment means for adjusting the power of theoutgoing light for the individual divided bands devided by the banddivision means.
 38. The individual band gain equalizer according toclaim 37, wherein said band division means divides said signalwavelength band into three bands: an SHB band, said gain deviation bandand an SRS band, said SHB band being under dominance of a spectral holeburning (SHB) effect as said first band, and said SRS band being underdominance of a stimulated Raman scattering (SRS) effect occurring in anoutput transmission line of said optical amplifier as another band ofsaid first band.
 39. The individual band gain equalizer according toclaim 38, wherein said adjustment means includes: a variable opticalattenuator for said SHB band; a variable optical attenuator for saidgain deviation band; and a variable optical attenuator for said SRSband, and said band division means includes: a gain deviation bandreflection device for reflecting light in said gain deviation band; afirst optical circulator for leading said incoming wavelengthmultiplexed light to said gain deviation band reflection device andleading reflected light from said gain deviation band reflection deviceto said variable optical attenuator for said gain deviation band; a bandseparation device for separating light passing through the gaindeviation band reflection device into light in said SHB band and lightin said SRS band to lead the lights to said variable optical attenuatorsfor said SHB band and said SRS band; a band coupling device for couplingoutput lights of said SHB band and SRS band variable opticalattenuators; and a second optical circulator provided on an output sideof said gain deviation band variable optical attenuator for leadingoutput light from said band coupling device to said gain deviation bandvariable optical attenuator side and for leading light inputted fromsaid gain deviation band variable optical attenuator side to an outputport; an SHB band reflection device, provided between said gaindeviation band variable optical attenuator and said second opticalcirculator, for reflecting, of light led to said gain deviation bandvariable optical attenuator side, light in said SHB band; and an SRSband reflection device, provided between said gain deviation bandvariable optical attenuator and said second optical circulator, forreflecting, of light led to said gain deviation band variable opticalattenuator side, light in said SRS band.
 40. The individual band gainequalizer according to claim 39, wherein each of said variable opticalattenuators is a high-speed variable optical attenuator having aresponse speed on the order of microsecond.
 41. The individual band gainequalizer according to claim 38, wherein said band division meansincludes: a first edge filter having a leading edge portion of atransmission band with respect to a wavelength in the vicinity of theboundary between said SHB band and said gain deviation band and having atransmission characteristic with respect to light with a wavelength on alonger-wavelength side from said leading edge portion; and a second edgefilter having a trailing edge portion of a transmission band withrespect to a wavelength in the vicinity of the boundary between saidgain deviation band and said SRS band and having a transmissioncharacteristic with respect to light with a wavelength on ashorter-wavelength side from said trailing edge portion, and saidadjustment means includes: a variable optical attenuator for adjustingoutput light power of said second edge filter; and a transmissionwavelength control unit for controlling a transmission light quantitiesof said SHB band and said SRS band independently by shifting said edgeportions of said edge filters individually.
 42. The individual band gainequalizer according to claim 38, wherein said band division meansincludes: a first transmission fiber grating having an adjustmentfunction to adjust a transmission light quantity of light in said SHBband contained in said inputted wavelength multiplexed light; and secondtransmission fiber grating having an adjustment function to adjust atransmission light quantity of light in said SRS band contained inoutput light of said first transmission fiber grating, and saidadjustment means includes: a variable optical attenuator for adjustingoutput light power of said second transmission fiber grating; and atransmission quantity control unit for applying a temperature orpressure variation to each of said transmission fiber gratingsindividually to control a transmission light quantity of each of saidtransmission fiber gratings independently.
 43. A wavelength multiplexingtransmission apparatus using an individual band gain equalizer,comprising: said individual band gain equalizer including: a banddivision means for dividing a signal wavelength band of an incomingwavelength multiplexed light into at least a first band and second band:said first band has tendency of decreasing power of out going light ofan optical amplifier per channel at a decrease in the number of signalwavelengths or increasing power of the outgoing light per channel at anincrease in the number of signal wavelengths and said second band, is asignal wavelength band other than this band, including a gain deviationband in which power of the outgoing light per channel varies mainly dueto a wavelength deviation of automatic gain control in the opticalamplifier; and an adjustment means for adjusting the power of theoutgoing light for the individual divided bands devided by said banddivision means, and control means for monitoring power of input light oroutput light of said individual band gain equalizer for each of thedivided bands to control the output light power adjustment for theindividual divided bands in said adjustment means based on a result ofthe monitoring so that output light power of each of the divided bandsbecomes a predetermined target value.
 44. The wavelength multiplexingtransmission apparatus according to claim 43, wherein said control meansincludes: an individual band monitoring unit for monitoring power ofinput light or output light of said individual band gain equalizer foreach of the divided bands; a storage unit for storing said target valuein advance; a difference detection unit for detecting a differencebetween a result of the monitoring by said individual band monitoringunit and said target value in said storage unit for each of the dividedbands; and a gain equalization control unit for controlling saidadjustment means so that the difference detected by said differencedetection unit for each of the divided bands reaches a minimum.
 45. Thewavelength multiplexing transmission apparatus according to claim 43,wherein said wavelength multiplexing transmission apparatus is arrangedas a wavelength add/drop apparatus having an add/drop unit made toadd/drop light with a wavelength forming at least a portion of saidinputted wavelength multiplexed light, and said individual band gainequalizer is provided at a former or latter stage of said add/drop unit.46. A wavelength multiplexing transmission system using an individualband gain equalizer, comprising a wavelength multiplexing transmissionapparatus including an individual band gain equalizer, wherein saidindividual band gain equalizer comprising: a band division means fordividing a signal wavelength band of an incoming wavelength multiplexedlight into at least a first band and second band: said first band hastendency of decreasing power of out going light of an optical amplifierper channel at a decrease in the number of signal wavelengths orincreasing power of the outgoing light per channel at an increase in thenumber of signal wavelengths and said second band, is a signalwavelength band other than this band, including a gain deviation band inwhich power of the outgoing light per channel varies mainly due to awavelength deviation of automatic gain control in the optical amplifier;and an adjustment means for adjusting the power of the outgoing lightfor the individual divided bands devided by said band division means,and control means for monitoring power of input light or output light ofsaid individual band gain equalizer for each of the divided bands tocontrol the output light power adjustment for the individual dividedbands in said adjustment means based on a result of the monitoring sothat output light power of each of the divided bands becomes apredetermined target value.
 47. An optical amplifier using an individualband gain equalizer, comprising: an amplification medium for amplifyingincoming wavelength multiplexed light; an automatic gain control unitfor carrying out automatic gain control based on power of incoming lightand outgoing light of said amplification medium and a target gain; aindividual band gain equalizer for dividing a signal wavelength band ofan outgoing light of said amplification medium into at least a firstband and second band: said first band has tendency of decreasing powerof the outgoing light per channel at a decrease in the number of signalwavelengths or increasing power of the outgoing light per channel at anincrease in the number of signal wavelengths and said second band, is asignal wavelength band other than said first band, including a gaindeviation band in which power of the outgoing light varies mainly due toa wavelength deviation of the automatic gain control so as to adjust thepower of the outgoing light for the individual divided bands; andcontrol means for monitoring the power of the outgoing light of saidamplification medium for each of the divided bands to control saidoutput light power adjustment of said band unit gain equalizer for eachof the divided bands based on a result of the monitoring so that thepower of the outgoing light of each of the divided bands becomes apredetermined target value.
 48. The optical amplifier according to claim47, wherein said control means includes: an individual band monitoringunit for monitoring output light power of said amplification medium foreach of the divided bands; a storage unit for storing said target valuein advance; a difference detection unit for detecting a differencebetween a result of the monitoring by said individual band monitoringunit and said target value in said storage unit for each of the dividedbands; and a gain equalization control unit for controlling saidindividual band gain equalizer so that the difference detected by saiddifference detection unit for each of the divided bands reaches aminimum.
 49. A wavelength multiplexing transmission system using anindividual band gain equalizer, comprising: said individual band gainequalizer including: a band division means for dividing a signalwavelength band of an incoming wavelength multiplexed light into atleast a first band and second band: said first band has tendency ofdecreasing of power of the outgoing light of an optical amplifier perchannel at a decrease in the number of signal wavelengths or increasingof power of the outgoing light per channel at an increase in the numberof signal wavelengths and said second band, is a signal wavelength bandother than said first band, including a gain deviation band in whichpower of the outgoing light per channel varies mainly due to awavelength deviation of automatic gain control in the optical amplifier;and an adjustment means for adjusting the output light power for theindividual divided bands devided by said band division means, and anoptical amplifier including: control means for monitoring power of inputlight or output light of said individual band gain equalizer for each ofthe divided bands to control the output light power adjustment for theindividual divided bands in said adjustment means based on a result ofthe monitoring so that power of the output light of each of the dividedbands becomes a predetermined target value.